Maybe, maybe not. What is the proof of the connection? Just the time of disappearance and the username? That is pretty circumstantial, but maybe. Not conclusive though.
I put very little trust in self-reported information online. Someone like Ghislaine would probably recognize the value and zero cost of sowing nuggets of false information in public posts. (However, this expectation also makes me doubt that Ghislaine Maxwell would choose a username with "maxwell" in it...)
Maxwellhill is an old account. I think online user anonymity is a biggest deal and people realize the value of that. Sort of like op security. So I could see her registering that 14 years ago. But over time she should have realized she need better security. Why she would continue to use it 14 years later is more of a question?
Maybe. The account stopped posting 2 days before she was arrested. And hasn't posted since.