My interpretation was that it all happened the way we experienced it. Penacony is less of a discrete series of levels between reality and the dreamscape and just more a gradient since the memoria was just leaking out into space.
No she's not the same Raiden Mei. She is another version of Mei on a different branch of the imaginary tree, who's world underwent similar events as the one in HI3rd. Just in Acheron's. it's implied Kiana died and then IX showed up and ate everything.
Question for those that played HI3rd. I haven't finished 2.2 yet (stuck on the final boss) but I assume this question can be answered safely with the story that's been revealed so far. Also I played up to chapter 32 of HI3.
Are we supposed to assume that Acheron/Raiden Mei is the same Mei from HI3rd, that ended up on a different branch of the imaginary tree? The conversation between her and Welt suggests both of them are the same characters as HI3
Yes, the implication is that Acheron's home world was a version of Earth as seen in Honkai 3. It was not a bubble universe, because there were no indications that it was unstable in itself, leaving another branch of the imaginary tree as the only remaining possibility.
No she's not the same Raiden Mei. She is another version of Mei on a different branch of the imaginary tree, who's world underwent similar events as the one in HI3rd. Just in Acheron's. it's implied Kiana died and then IX showed up and ate everything.