FTL Fusions
So I don't see any means to escape the conclusion that given mereological universalism, some things trivially move faster than light. Lots of things, in fact. Perhaps that's less troublesome than I thought because these things don't actually violate any physical laws.
For instance, I guess the principle that physics looks the same for all things that move with constant speed relative to each other has to be restricted to things with speed < c anyway. (At least Lorentz transformation doesn't make much sense if v = c.) If so, the exclusion of faster-than-light fusions from the principle is already built in and we don't need to worry about e.g. what such a fusion's proper time might be.
We might still worry about momentum and energy. Consider a gerrymandered but continuous fusion that moves with speed c. Some (instantaneous) stages of the fusion might look like they have positive rest mass, e.g. a stage it shares with an electron. So at this time do we get a thing with positive rest mass moving with speed c? Perhaps not, because such an instantaneous rest mass simply doesn't count. For couldn't we just as well say that as long as the fusion has the mass of the electron it exactly follows the electron's path, which is slower than c. The point is that the velocity of an instantaneous timeslice is undefined. Only extended segments have velocity. But no extended segment of our fusion has positive rest mass. I'm not sure if this trick goes through, but it seems to me that it could.
If it does, it turns out to be important that our universe doesn't contain any dense regions extending in more than one dimension such that every point inside of it has positive rest mass, like the famous spinning sphere.