You can't protect D17 from her mother's actions, unfortunately.

Just do what you're doing and be there for her yourself.
Be very careful NOT to bad mouth Mom.
Just be her sounding board.

If you have to rip wife's face off at some point, do it on the down low.

Be very careful not to do anything that could make D17 feel like she's in the middle.

Spend time with your daughter.
You can include her in your GAL efforts.

Besides, she'll be gone before you know it.

So enjoy this time.