Elizabeth was John’s co-conspirator for years. His partner in crime. She loved the millions and being very rich and being along for the ride. I would never rate her as someone with high morals or high character. She was as bad as John. She probably gave him a lot of his lawyering ideas.
Elizabeth was four years older than John. This tends to create a family dynamic where the wife feels she should dominate. And John being younger made him more prone to wanting something new and exciting in women....an affair
I agree with your first paragraph but not your second.
My sister is married to a younger man and there is none of what you describe. It depends on the couple. I think it was her illness that he was escaping from since he was the weaker of the two.