What makes the border of the universe and can it change?

[JesseM]

In both cases, BH's and Cosmo Event Horizon, objects allegedly still exist somewhere, though not "accessible" to us.

Not if the black hole shrinks down to nothing as Hawking radiation escapes it, and the Hawking radiation itself is completely random. That's the source of the "black hole information loss paradox"--again, information loss does not just refer to information becoming inaccessible, like when a galaxy crosses our cosmological event horizon, it refers specifically to what happens when a black hole evaporates via Hawking radiation. Some physicists seem to think the resolution of the paradox is that Hawking radiation is not truly random, that one way or another it manages to encode the information that disappeared over the horizon...there may be other types of proposals as well, I'm not sure.