[Professor Mega-Mind (OP)]

          Planetary Spin-down
 What reduced the rotation rate of the inner planets ?  We know that the Moon is primarily responsible for Earth's slowing , but what about Mercury , Venus , and Mars ?  All must have started with a rapid rotation rate.   Something(s) braked their spin .  If it was powerful solar-tidal effects , how is the Earth still spinning fast , when Lunar-tidal forces are much stronger ?  Some other effect is at work ; an  extremely strong , and long-lasting effect . It must be crustal rigidity .  Dry planetary crusts & mantles are much more resistant to deformation than wet ones , therefore they resist it much more .  This converts tidal bulging into heat , by way of more tidal drag .  Mars is less dry , but more cold (frozen) .  Similar effect .  Planetary surfaces therefore profoundly affect planetary rotation rates .
Critiques please ......P.M.

**Addendum : A more technical analysis of this subject can be read in my (D.H.) post , Quora thread : Why does the atmosphere on Venus move around at such high speeds when the planet rotates on it's axis so slowly ?
www.quora.com/Why-does-the-atmosphere-on-venus-move-around-at-such-high-speeds-when-the-planet-rotates-on-its-axis-so-slowly?
 *Use the link there to access the foundational work of Doctor.PhD Jerry L. Krause , Chemistry : Concentration in Atmospheric and Climatological .
》Ref. : N.S.F. - How do proto-planets form ?  Reply # 14 . 
www.thenakedscientists.com/forum/index.php?topic=74806.new;topicseen#new 

[Janus]

The time it takes for a body orbiting another body to tidally lock is found by the equation:
T = wa^6 mQ/(3GM^2 K R^3)
w is the starting angular velocity of the body's rotation
a is the semi-major axis of its orbit
Q is the dissipation factor
m the body's mass
G the  universal gravitational constant
M the mass of tht body it is orbiting
K the "Love" number.
R the radius of the body.
Q and K are not well known except in the case of the Earth and Moon system.  So for the sake of argument, we will assume they are the same for all three planets and the Sun.
Since we are comparing planets orbiting the Sun G and M will be the same.
If we assume that w starts out as the came for all three then the difference in tidal locking times can be simplified to the relationship of
a^6 m/R^3   
It also turns out that the ratio of m/R^3 comes out to be fairly close to each other for all three planets.  So in the end, the major deciding factor is a^6
 Earth is 2.58 times further from the Sun than Mercury, so it would take nearly 300 times longer to tidal lock to the Sun.
The Earth is 1.38 times further than Venus and would take 7 time longer to lock.  Assuming all other things are equal.

One thing to keep in mind is that tidal forces fall of by the cube of the distance.  So even though the Moon's tidal effect of the Earth is ~ twice that of the Sun's, At the distance of Mercury, the Sun's tidal force has increased by a factor of 17, and be 8.5 times stronger than the Moon's effect on the Earth and at the distance of Venus it would be 1.3 times stronger than the Moon on the Earth. 

But we don't know that all things started out equal or stayed that way. For example, the Earth is believed to have been struck by a Mars sized body in its past which initially formed the Moon, and likely spun the Earth up quite a bit.  This would have given it a lot more rotational energy to shed.
Venus actually rotates slower than it orbits.  This also may have been due to a collision; one that robbed it of spin rather than giving it more spin. 

So without knowing the full history of each planet it is hard to say what all the influences were that contributed to their present rotations.

[Professor Mega-Mind (OP)]

         Mr. Quantitative-Janus
 As always , you formulate quite impressively !  Your equations have convinced me that Mercury and Venus have slowed unusually quickly , due primarily to their extremely-stiff crusts . Venus' tidal-bulge should be ~ 3/4 that of Earth , while Mercury's should be about 5/4 of it . I have 2 qualitative issues that need quantitative analysis here .  First up , Mars .  How in the blazes is that tiny little rock , with minimal metals & core , still volcanic ?  Could it be heat from tidal deformation of that cold crust & stiff mantle ?  Next up , Venus .  The Sun's gravity is 2 times stronger here . That is one heck of an atmospheric tidal bulge in that super-rotating , super-dense massive atmosphere .  It not only puts drag-torque upon the surface, it also pulls upon the massive land tidal-bulge gravitationally .  Did this double-whammy stop the initial spin ?  Did the extra-stiff crust and mantle ?  Answers , brain-people , I need answers !..........P.M.
...................Question
Why was the Moon lifted to a much higher orbit , by Earth's tidal/rotational forces , while Io was not lifted by Jupiter's ?
Putative answer : The other Jovian moons fight to maintain the gravitationally resonant orbital pattern present in system . This then translates Jupiter's lifting effort into tidal-heating of the moons , instead of into orbital height .

[Kryptid]

In the case of Venus, it is not only rotating slowly, but also in reverse (relative to the other planets). I believe the current explanation for that is that it was hit by a large object (like a protoplanet) in the past that altered its spin.

[Professor Mega-Mind (OP)]

              The Hokey-Pokey
 Really Mr.Kryptid , I'm surprised at you !  You know , it's not always a giant impact .  What are the odds that a monster impact would just precisely cancel out Venus' spin .  If you believe it is tidal , what are the odds that we are here to see it  ,  just when Venus tide-locks ?  Last but not least , what are the odds that Venus tide-locks at all , since it definitely is not slowing down (except for the below) ?  In light of those incredible tidal-forces you mentioned , shouldn't  Venus be slowing down even faster than old Earth is (aside from cyclical variations induced by atmospheric & geological occurrences , and orbital-cycles) ?  There must be  some-thing  propping Venus' rotation up , besides the memory of a GIANT IMPACT ! 
Hokay , had fun , bye .........P.M.
.................Addendum .
Any giant impact is just as likely to spin up an impacted body , as to spin it down .  The over-whelming odds , are that the spin  is not affected to an extreme degree .
.🤔
Ref.: Milankovitch-Cycles .

[yor_on]

The problem with wanting specific answers astronomically is that nobody knows it all Mr Megamind. And mostly you have a lot of unknowns. After all, how long have we been here? And how old is the solar system we exist in? What we can do is like Janus did. which is take what we do know and apply it on the question. Which, if I may say so, was pretty impressive to me.
=

And for you Janus I'm going to make a exception, I'm going to 'like' your answer. Which I normally hate. It breaks down a conversation into likes and dislikes. But I'll do it.

[yor_on]

Damn, I don't see how to do it?
 heh, found it.

[Professor Mega-Mind (OP)]

..............The Janus Factor
As I said , his formulating is most impressive .  I would , however , question whether tidal force increases by the cube , since gravitational force increases by the square .
Go ahead , Gridley !...........P.M.
..................Addendum
Talk about counter-intuitive !  The difference is because tidal/gravitational attraction is 2-dimensional , whereas tidal/ rotational friction is 3-dimension -al . 

[yor_on]

Maybe you mean why the difference?
http://austides.com/wp-content/uploads/2014/06/Myths_Sawicki.pdf

"  Note that in the second step in Eq.(3) we neglected corrections from higher order R/ds terms. A similar calculation shows that the gravitational  pull of  the Sun per 1 kg mass at the point on Earth closest to the Sun is in turn greater than
as, also by the amount ∆as. This result could be also obtained by taking the differential of the gravitational force equation. Note that ∆as is inversely proportional to the cube of the distance. "

[Kryptid]

              The Hokey-Pokey
 Really Mr.Kryptid , I'm surprised at you !  You know , it's not always a giant impact .  What are the odds that a monster impact would just precisely cancel out Venus' spin .

It didn't nor did anyone say that it did.

If you believe it is tidal , what are the odds that we are here to see it, just when Venus tide-locks ?

We aren't just now seeing Venus tidally lock. Its spin is currently slowing down: Over a period of 16 years, it was found that a day on Venus increased in duration by 6.5 minutes, likely due to atmospheric friction: http://www.esa.int/Our_Activities/Space_Science/Venus_Express/Could_Venus_be_shifting_gear In order for it tidally lock, its spin would have to increase instead, as day on Venus is currently longer than a year.

Last but not least , what are the odds that Venus tide-locks at all , since it definitely is not slowing down ?

Oh yes it is. See above.

In light of those incredible tidalforces you mentioned ,shouldn't Venus be slowing down even faster than old Earth is ?

Its rotation is slowing down faster than the Earth's is. Again, see above.

[Professor Mega-Mind (OP)]

......Same Page - Different Side
We are actually in agreement .  I will now illustrate :
A. We agree it was probably not a giant impact . Way too simple .
B. Venus is slowing it's rotation rate because it's revolution rate is slowing .  This must happen as the planet is tidally lifted further from Sol .  Other factors are mentioned below .
C. Venus tide-locked over a billion years ago , then was dragged into retrograde rotation by tidal and atmospheric effects .  The balance between these dragging forces , and crustal friction , assures that no matter how long the Venus year is , the " day " will always be 18 terran days longer . 
Good luck finding to-the-second , historical measurements of Venus year length .
............P.M.
..................Addendum.
The rotational slow-down of Venus observed by Messenger , and the Venus Express orbiter , is now being attributed to changing heavy-atmosphere air-flow patterns .  There is also an effect from crustal and mantle movements and plumes , as well as an alternating Milankovitch Cycle (tidal-bulge) .

[Kryptid]

A. We agree it was probably not a giant impact .

I never said that.

B. Venus is slowing it's rotation rate because it's revolution rate is slowing .  This must happen as the planet is tidally lifted further from Sol . 

Source?

C. Venus tide-locked over a billion years ago , then was dragged into retrograde rotation by tidal and atmospheric effects .  The balance between these dragging forces , and crustal friction , assures that no matter how long the Venus year is , the " day " will always be 18 terran days longer .

Source?

[Professor Mega-Mind (OP)]

...............Venus Rotation
First , orbital distance .  The tidal interaction between Sol and Venus should be far less than between Earth and Luna .  The basic separation , however , is far more .  My guess-timation for tidal lift is 2 million kilometers .  The 6 minutes you mentioned is likely due to geological , and
atmospheric processes , in particular ; crustal uplift , and mantle plume ascension , or heavy-atmosphere current changes .  These would move planetary mass outward , slowing planetary rotation in a manner akin to a twirling skater extending their arms .  Such processes would play out over many millions of years , causing the Venusian day to alter in length on a geologic timescale .
 Next , the tide-locking scenario .  The Venus Express Mission established a likely ocean loss time of  ~2 billion years ago .  Add another billion for the crust to out-gas it's H2O and become stiff enough to resist rotation , and you've got spin-down & tide-lock  ~1 billion years ago .  The caveat being that the atmospheric super-rotation , and tidal torque , were strong enough to drag the planet into a slow retrograde rotation .
There it is ; as long as there is solar energy to power the atmospheric winds , Venus will struggle to overcome it's extra-stiff crust , and slowly rotate backwards .
Well , there ya have it ! 
Unnecessary zoom !
......P.M.

[Kryptid]

Sounds like you're doing a lot of guessing to me.

[Professor Mega-Mind (OP)]

..............Orbital Mechanics
Mr.k. , these are exciting times for astronomy & planetology !  There are actually several reasons posited for the observed widening of the planetary orbits .  Try stellar mass loss , core contraction , and Sun/planet tidal interactions .  A good reference source for this is SPARKONIT.Com .  Read their Jan. 25 , 2018 article : The Orbits of...  , in the Space section .  NASA Goddard Space Center also has excellent source material on this .  If you want to see more debate on these subjects go to Astronomy Stack Exchange . 
Now , in regards to Venus' slowing rotation .  You could research  the effects of geological , hydrological , and meteorological mass movements on planetary rotations .  Alternatively , you could study basic rotational dynamics , and apply it to the dancer scenario I mentioned previously . 
Bottom line ; it's all established processes , and high-quality estimates . 
Okay , you have the facts , now know the patterns .
......P.M.
Addendum : The fundamental motivator for the Venusian atmosphere's westward rotation , is a combination of solar influences . These would be the  solar-wind , and the solar EM flux  .  Both carry significant angular momentum from Sol's rapid , counter-clockwise rotation . The reflection /absorption of these transmits a gentle push to the planet , in the westward direction  .  Ionization of the ionosphere allows for Compton Scattering to  occur also .  The striking x-rays  transfer their angular momentum to the relativistic electrons , which transfer it to the atmosphere , etc . 
The closer a planet is to a star , the stronger this push is , but the tidal forces involved increase far faster than that .  Add in glacial-stabilization , and you wind up with days over a century long .
Exo-geology MUST take this into account , when analyzing the geological history and processes , of accessible exo-planets .
D.H.

[Professor Mega-Mind (OP)]

The above discussion brings up a relevant subject , with significant bearing upon the processes referenced above .
Tidal-lifting is the subject ; why do some planet/moons experience it , while others do not ?
Both Mercury and Io are subject to very strong tidal-lift forces , similar to Luna . Neither should be able to maintain a stable orbit , in a numerically resonant relationship with it's co-orbiting satellite bodies.  The fact that they do is indicative of a regularizing mechanism , one capable of keeping satellites in stable orbits for billions of years , even in the face of powerful destabilizing effects . The physical evidence strongly suggests tidal-deformation , generating extreme internal heat . The lifting-forces at Io are the more extreme , as are the gravitational interactions with the other Jovian satellites . The power of these processes is apparently the cause of Io's incredibly violent volcanism . The tremendous force exerted upon Io by Jupiter , does not succeed in perpetually lifting that moon into a higher orbit . Rather , it is expended as volcanic heat , from Io's surface .
This same process affects planet Mercury , but to a much lesser degree . The tidal forces involved are much weaker , as are the gravitational interactions between Mercury and Sol’s other planets . The net effect upon Mercury is significant mantle heating ; not enough to cause planetary surface volcanism , but enough to allow for deep-mantle liquification and convection . This would be the source of Mercury's planetary magnetic-field . The planet is too small to still have great internal heat , unless there is a substantial source of extra thermal energy .
The above-referenced process is actually evidenced throughout the solar-system , in a number of moons and planets . Eventually , it should be observed in exoplanets under observation , across the heavens , once observational equipment and techniques are improved , and become sensitive enough to make said observations.
P.M.
*Read NSF. Thread :
How do protoplanets form ?
Address : Top-paragraph .