re: So, who was really at fault (or most at fault), in Mrs. Doubtfire?

Miranda was moving forward in her career and fell in lust with Pierce Brosnan who was wealthy, nice, attractive and seemingly goal oriented. Plus she had a history with him. Maybe she was always in love with him and settled for Daniel.

Daniel was the fly by the seat of your pants fun parent who didn't seem very motivated.

I guess coming home to a party she wasn't aware of mixed with the fact that her husband just quit his job didn't sit very well with her so she did what she thought was right for her and her children.

The custody hearing seemed like it went pretty normal, with the father usually getting the short end of the stick in regards to visitation. I mean, he didn't have a job at the time and barely a place to live so it wasn't that extreme. He was desperate so he went full on crazy adopting the Mrs. Doubtfire character knowing that if he was caught it could mean nearly no contact with his kids, which is pretty much what happened.

Miranda, after finding out that he lied to pretty much everyone for a substantial period of time, and was cross-dressing to babysit his kids had to draw the line somewhere.

She did love him though and eventually came around after some time apart. She knew that he loved his kids more than life and had paid the price for what he had done.

In the end, they both gave up on each other and put their kids before their marriage, so I would say they were both at fault initially. She made the hard/selfish choice to divorce, he made the crazy choice to deceive everyone.