So what you saying in a nutshell is a massive object cannot attain c relative to an observer. So my earlier statement holds: "Mainstream science does not theorize that masses are accelerating away from each other through space, for one thing that would violate SR", but only for an observer 'on' one of the masses, which was the context of the statement I was addressing.
Closing and thus parting velocities can exceed $$c$$. My original post on this quoted a signle sentence and was meant to address only the accuracy of that statement. I was not commenting on the expansion of the universe generally.
I do have some reservations as to whether it is possible that complex matter (matter composed of atoms) can be accelerated, from a relative state of rest, to relativistic velocities. The reason(s) for this stray from the initial discussion. And I don't believe it would add anything, other than further complexity to the current discussion.
However, in the case of charged particles, the situation is fairly straight forward. They can be accelerated to relativistic velocities both naturally and artificially in accelerators, like the LHC.
Back to your original discussion, involving the expansion of space. When we look out into the universe in any direction and see that objects, at least appear to be accelerating away from us, and that — that acceleration increases proportionally with distance, the velocities of the objects we are observing, would seem to be subject to the restrictions, for the additions of velocities within the context of SR. However, this does assume that space as the medium through which these objects move, is in fact uniform. (I do not use the word medium here as any reference to substance or any ether, only that we do have evidence that space is a dynamic counter part, when observing the relationships of objects within it (space).)
This qualification is based on the fact that all we know about space is defined in a relatively localized frame of reference. Everything we know is from how things in a gravity well behave. In fact the evidence for expansion and accelerating expansion all originates from, how it appears that space between galaxies and more specifically galactic clusters, seems to be, behaving. Gravity appears to be dominant above some threshold of matter density associated with gravitational inertial systems and not so much between those gravitational inertial systems. (To be clear, I use the phrasing "gravitational inertial systems" to define solar systems, galaxies and galactic clusters...) It maybe and seems likely that to some extent the helosphere of our solar system represents the boundary between the solar system's inertial system and the Milky Way galaxy.., and it would seem likely that similar boundaries exist, for galaxies and galactic clusters.?. And that between the boundaries of galactic clusters, space/spacetime appears to be behaving differently than within a gravity well.
Since even though space cannot within the context of GR be completely homogenous, or the same everywhere, SR cannot apply in exactly the same way it would in a hypothetical. Still, it does seem more reasonable to me that space is expanding, than that objects are receding with velocities that approach or even exceed $$c$$. The mechanisms for that expansion remain a mystery and necessitate the need for the inclusion of unknowns like dark energy, to reconcile GR with observation, at those cosmological scales.
Basically what I am saying is that, IF what we understand of SR and GR from our locally defined perspective, can be accurately applied to the universe generally, space appears to be expanding, even at an excellerating rate. On the other hand, it could also be that at some time in the future we may discover that just as Newton's field equations are a locally consistent approximation of GR, SR and GR may be similarly locally defined approximations, when viewed at a far larger scale.