I think it's pretty clear that Kris = altChara of some sort. Kris is definitely implied to have the behavior of Chara. The only thing I couldn't figure out is why Kris would be all about sparing in the dark world. But that's just because you are controlling Kris at the time. So the question is, why is Kris allowing us to control them like that? Did Gaster put us in them, and then they only managed to get us out in the ending? The bird cage already being there implies that this happens fairly often though.
Also I do think the imaginary aspect of the dark world is really important. The outfits and flavor text and new battle UI and everything for sure mean it's not a real place. But then what's up with Ralsei (Kris imagining his brother?) and with Susie sharing the dream? It was a fully contained story that doesn't need to be readdressed, so I wonder if future chapters will just be more imaginations, maybe with different characters? ---