I haven't worked on the game in over a year, if/when I ever do again, it'll probably be a total remake/overhaul but I'll try and answer your questions.
I forgot the criteria you need to fulfil for all the skins but one of them is beating a custom night with that character set to an AI level of 20, and the other is for completing a custom night with everybody set to an AI level of 20.
There's meant to be an ending after night 6, with night 7 being the custom night, if it keeps counting up then it's just bugged, apologies, it was my first attempt at a game that I actually completed.
Is Bucky bugged in the custom night? If not, then its probably because the normal nights have preset AI levels and if it's going over the amount of nights I specified the AI levels for it might just be using the default/debug ones I made for testing everyone.