Very nice one. if we assume A and B being at rest with each other and also assume that they are at a uniformly accelerating pace at f.ex one Gravitation, then the equivalence principle say that they will observe it just the same as if they both were on earth.

If we then assume an observer C at rest versus the place A and B once left (Mars:) somehow observing that signals journey through space, he should see it take a much longer path as the vectors would be different from his frame of observation. Not only that, it would also according to him take a longer time than that second those two ships would measure. So who would be right?

Both would be right. That's the new paradigm of relativity, that distances and time are relative the observer. One way to look at it is to assume a plasticity to SpaceTime, not only to time but also to the 'distances' in it. And what changes it is gravity/mass, and acceleration that then becomes equivalent to mass (relative mass/momentum).

There is a hidden 'gold standard' though. you can fit the observations to each other if knowing both frames observations, that as the 'distant observer' will observe the acceleration even if A and B would be inside 'black boxes' themselves, so assuming that his communication can reach them there will be a possibility to fit their observations into a whole. Well, as I see it.

And another thing, that light traveling the longer path, according to our 'distant observer'. Wouldn't it have a red shift too, as observed by that 'distant observer'?