WARNING! UNPOPULAR FNAF OPINONS AHEAD! (Gregbot)
I like the concept of Gregory is a robot and that Glamrock Freddy could be possessed, not so much that Gregory is Dave/The Crying Child & Glamrock Freddy is Michael. Might just be me wanting all the aftons to stay dead, but I believe Henry and Edwin to some extent have just as much reason to be GL Freddy as Michael does. They both fit the narrative of not being able to protect their lost children, while also connecting them to fires. My friends are here referring to the animatronics they built and felt a kinship to, while 'she' could be M1/F10-N4. (Plus, a Dad and his son are much more fitting to GL Freddy and Gregory's situation than an older and younger brother.) As for Gregory, yes I do agree he has some sort of connection to the 3 other characters named Dave/David in this franchise, I'm hesitant to say that he is one of them. Although, I think I prefer that he's just M2 after the secret ending of SOTM, and that's how we find the mimic in ruin. Like what Candy Cadet said 'The mother caught the monster and kept it locked in the basement', this implies that the mimic (the monster) was free (escaped the pizzaplex) until either Vanessa or F10-N4 caught him and locked him up in the basement. In addition, because the mimic utilizes one of the very first lines in Security breach said by Gregory I believe it's safe to assume that M2 was somewhere in Freddy's greenroom at the start of the game, so why not just be Gregory himself, or even Freddy for that matter? Also, if there was ANY character that could make themself so compact as to fit in Freddy's stomach hatch, it would be the mimic. Kinda explains how we can see through Freddy's eyes when we pilot him around too. Gregory essentially being a refashioned M2 could also explain the design differences between Ruin mimic and SOTM mimic. Michael aftons room smack in the middle of the pizzaplex would still be kind of a loose end though. Maybe its Michael's message to Gregory or perhaps someone else to destroy the GL animatronics and save those with soul?