I love this so so much. Very pretty and very interesting game that literally got me hooked 'till the last ending.
Okay, so,i guess those are gonna be!SPOILERS!
About the ??? ending:
So, it was really hard getting the ??? end, but thanks to the guide and another commenter i figured that the "feed the mouth something else"(or something like that) also counts as another ending. So i did it, and surely enough, something changed in the next game-there was a fire exit door, so i obviously clicked it, and then there was this black screen with a looping sound of footsteps (on the stairs ig). I read that some commenters had the same issue so my guess is that either this is a bug that still isn't fixed (which would be kinda weird, but alright) or this IS the ending-MC finally, as it seems, having a chance to escape when in reality there's no escape, so he just runs and runs down the stairs searching for an exit. Forever. Kinda depressing, but, i mean, that's what you should expect from this game.
I'm not sure about this theory though, since theres no sticker or name to that "ending". But yeah, that was FUN.