In the response to the question, "is there a special inertial reference frame in space?" My opinion is the answer is yes, but it's a little tricky to see at first. For example, you could take a "consensus" of all the objects in the universe. You could pool up all the velocities and masses of every object, and find a one true "average momentum" of everything. There is some velocity out there, that would represent the lowest possible momentum. That is, there is a certain velocity that if you traveled any faster in any direction, the new summation of momentums would have to be greater. You could call that state, the inertial frame if you wanted.

I think this idea is backed by the fact that the strength of the cmbr is dependent on your velocity relative to the motion of the cmbr. Thus cmbr IS frame dependent.

To your second question, is the inertial frame of reference the same everywhere? I'm not sure what modern physics says about the question, but I know what my theory says (and I'm a quack so you can read on if you wish). My theory says that the inertial frame of reference (which is what determines the speed of light in my theory) is dependent on the summation of all objects momentums divided by the inverse of their distances. Since the acceleration and density of the universe is largely homogenous, you would find that the velocity of the inertial frame varies smoothly with distance. That is, the further away two objects became in distance, the more their average momentums would be pointing in the opposite direction. I think, The speed of light, and all the other relativistic effects are dependent on this frame. So the new speed of light would be c + the speed of the inertial frame at that specific location.

This is how I reconcile the fact that objects that are very far away from earth, and likely would have very high receding velocities, wouldn't be time dilated the same way as two objects with more similar frames (objects that are close to one another). To be more specific, time dilation would be determined, by the relative velocities of two objects, relative to each objects frame at that specific location. So its (relative velocity-frame velocity) compared to the others (relative velocity-frame velocity).

So if two objects were moving away from each other at .7c, but they were both at rest relative to the other objects in their area (thus very far apart in space) they would view no time dilation. If they were traveling at .7c and they were a mile apart (which in normal scenarios means they exist in "almost" the same frame) then time dilation is witnessed. And lastly, if an object was traveling at.9c from an object in an area so far away that most objects were traveling at .7c then the time dilation would be that of an object traveling at .2c, relative to the other.