...
I am trying to support daughter as much as I can. I am posting to seek your opinion about some things to do or not to do as I navigate how I can support my daughter. I really appreciate your experience and opinion.
Thank you.
The most important thing you can do is be there to listen to what your daughter says.
If you're with your daughter and your wife is not present, you can be more open with her. I still would not mention BPD, or bad mouth your wife, or anything like that, but you shouldn't also defend her behavior toward your daughter, and validate your daughter's feelings about it.
If your wife is present, and your in the midst of conflict, it's more challenging to negotiate, but you have to be careful not to take sides, and try to defuse things without making your daughter feel unsupported. She's also going to be looking to you for guidance, and if you're doing one thing and saying another, you're going to lose credibility in her eyes.
It's not going to be easy, but if you consider your words and actions carefully, you can navigate this issue without losing your daughter's trust, or escalating the fighting; that I think are your two goals when in the midst of it.
it sounds like at least you don't have to worry about placating your wife if you're already planning an exit from the marriage.