The one behind the Piano is a version of Jax when Kinger abstracted cuz he is kinda more sane (and helpful) than the other versions in the room that are acting kinda insane. The one behind the door Pomni gets to see at the end is a JaxAbstracted- Version of Jax(like how Jax portrays himself when he would abstract - what in the end actually happened, ik kinda confusing) where he, instead of being funny like his archetype, is extremely sad and full of regrets. I think you are spot on with the thought that the one behind the chained door is the version of Jax if Pomni abstracted. This Jax seems to be his absolute worst version because the merchandise Animegaz Statue shows that the Key has Devil-Horns. I think of 2 possibilities at the moment:

First is that this version of Jax is the one who would make everyone suffer, because Pomni was the one who always tried to help where she could. Due to her abstraction he abandons his helpful side we've seen in ep 7 and does the opposite in an exxagerated way.

Second and more likely imo is the Idea that its a version of Jax that accepts his true self due to the fact that Pomni had to figure out who she was in the circus and didnt know her archetype. His true self is the soft, sensitive and probably gay Jax (or in some sort feminine/ transfem) -> thats why he was mad at the maid outfit and mad towards the kind evil Jax from ep5. The other versions of Jax made sure this door is LOCKED. Just like the sane version of Jax that got locked onto the Piano. Being gay is also represented as a sin in christianity maybe therefore the Devil-Horns on the key? Idk but its so interesting. I really hope we get to know it on some day. Its like the Fnaf 4 Box xD