Ah, so he isn't completely alone nor that he is in stasis once crossing into the mirror world. So, why exactly does someone need to enter the mirror world for the spell to work anyway? So that Mirroro isn't alone anymore, making Mirroro, and subsequently the mirrors, stronger or something like that? I feel like it's a weird requirement made for drama, is all.
I generally chalk the requirement up as the "It's magic, I ain't gotta explain shit." factor (well, and "it's drama..."). Maybe you could explain it as how, just as the palace was still reflected by the lake after the spell went into effect (and thus could be said to still exist there?), you needed someone "real" to pass into a reflection, to take the palace with them? I think the reason Crowe volunteered and was determined to see it through, rather, was because he aspired to be that heroic figure he originally saw in Garron (and which he got a chance to see again, right before the spell finished). Rather than someone who might resent that fate, or feel trapped, he wanted to ensure Mirrorro had a "friend".
Speaking of Momoslow, I thought his name was actually Momo or Momoslo and that he was called Momoslow because he's, well, slow (which isn't true). Guess that's really his name.
The katakana is "mo-mo-su-ro", so Momoslo could've worked but honestly, it looked like the word just sort of cut off abruptly, like it was missing a letter, so I included the "w" in the western spelling (similarly with Crowe's name, he's "ku-ro-u", but "Crow" would've been a weird name for a rabbit, and I felt like adding the "e" makes it readable as being just a little extended, like the original).
Thus far, there haven't been any official guides to how the names should be romanized, if that ever happens, the adjusted names will be used (and earlier chapters might even get corrected).