I'll bet it's different with you guys fully split now & staying with dad will closely mimic their comfort zone.... until there's an event of some sort. Definitely his overcompensation plays a part - so does the fact that he's a pushover compared to you. It's the path of least resistance, isn't it? Especially now that there isn't the stress of dad off on binges, mom upset, divorce details to be worked out.
From their perspective, I actually get it. They are thinking of the problems between you guys are YOUR problems, not theirs.... they do not understand all of the insidious ways that they've already been affected so don't see anything wrong with what they're doing. What they aren't seeing is that this isn't just Marriage Problems, it's a Family Disease.