Only in theories where it collapses, and nobody seriously thinks Copenhagen is correct these days.

Not quite. There are two speeds in relativity: the maximum speed and non-maximum speeds (i.e. everything else). It just so happens that light travels at the maximum speed.

Supra-luminal signals will almost necessarily imply that it's possible to violate causality by arranging a series of supra-luminal communications. Not every supra-luminal communication violates causality, but it's hard to think of a way to build a consistent theory that would both preclude causality violations and permit at least some supra-luminal signaling.

No. It's an absolute, inviolable, universal limit on any and all interactions in the universe. It was recently confirmed that even gravitational waves are limited to the speed of light. This (and the Fermi Paradox) are the strongest evidence that faster than light travel or communication of any kind is impossible.

We just call it the "speed of light" because it's the speed that massless particles (like photons) travel at in a vacuum.

Per relativity, any medium that could conceptually facilitate transfer of information or of causality (which are basically the same thing) faster than the speed that light happens to travel at would also be able to reverse direction and arrive at their source in space earlier than the time that they left. That effect is completely unrelated to light itself.

IIRC It’s not that light is a medium. It is that light goes that particular speed max (photons can go slower in fact) because that is the speed of causality.