This the first real time when Gojo and Marin are at odds. My bet it will be many, many chapters of slow and badly written drama. Marin propably already contacted some agency about cosplays and won't tell Gojo real reason for not wanting his cosplays to not make him feel guilty (just how No-chan...
No, small Gojo went to live with his cousins after accident and she was a neighbour of said cousins and in turn his as well for that time.
Tho even if she was his cousin, manga has been playing it in such a way with her that I would still expect them getting together to be acceptable in the...
We are almost at the worst case scenario. We know only need Non-chan to realise Marin isn't Gojo's girlfriend, get charmed by Gojo and either ask him out or make Marin realise that this girl likes Gojo and know (as I assume it will go) wants to support him in his dream of making hina dolls...
Maybe copy it with Clara's ability and burn copies. Or it was an illusion of sorts considering the fire they had was given to them and not of their own making