This is my assumption as well. Chekhov's Idol vanished too suddenly for her to not be back.
she's also the one who told kitami he should become a monster and the only person who'd know all the things lilly/lally wouldn't have known. It's kinda fucked up she did a 180° and became an alien lover for a second there
I'm still putting my chips on lally taking lilly's identity after he died. there's something to "his beloved big brother is all he has". if lally loses his brother, that line makes a lot more sense
Purely speculating and might be talking out of my ass but lilly has 1 freckle under his eye, and lally has 2. that's a really specific detail to include that's been repeated over and over. If I were the author and I wanted to reveal the brothers were flipped, suddenly revealing a second mole would be a better way to do it than just saying it