Is Miss Havisham related to Mr. Havisham?

It’s not a popular name that I’m aware of but it doesn’t seem that likely that she was using it as an homage. There doesn’t really seem to be any reason for her to have but you’d have to do some biographical research into her life to really verify whether there was or not.

I think they were fooling around in cockloft.

I don’t have any relevant knowledge (other than having read both books), but I’d say go right ahead and think so. It’s quite possible. Authors don’t have obvious or documentable reasons for everything they do. Maybe Burnett was indulging a fancy of her own.

I know that in some of my own fiction writing I have enjoyed making little secret allusions to certain people and things just because I could. Incidentally, only, and not in any way overindulged, but I did do it. In one story, I called a bad guy by the name of a kid who’d plagued me in fourth grade. No one would ever guess that from any personal history of mine. It was just my little silent revenge.