Responses
I don't think that would be wise to leave, cause itadori ate about 15 fingers (as jogo said )which means only 5 are left, so if he leaves for another vessel he could get strong, but never as strong as he is in itadori's body right now. So it's most likely your second option, he will probably take over itadori's body.
There are still fingers left right?? This smile could be that someone else ate his finger and resisted it like itadori ╮(￣▽￣)╭so he gonna leave itadori orrrr he gonna make a contract with itadori again