It won't be funny if she loses custody- it will be heartbreaking for DD. Even though she is BPD and not a good mother right now- it's sad. I say that because even though you'll be a good dad- there is just something about mom- and you never get over wanting momma to love you like she should. I'm 41 and I've never gotten over it.
Speaking as the daughter of someone who was borderline I doubt very much she is scared of the evaluation- because remember- they don't believe anything is wrong with them- it's everyone else.
She's probably indignant that she would have to be subjected to them. And she probably feels like she won't have to be.
Example- I'm a Christian- my mother pretended that she was. I believe everyone goes through judgment in Heaven. Even if they are saved. My mother was ill and dying from emphysema. She told me "When I die I won't have to go through judgment like everyone else because I have suffered enough on earth" And she really believed that kind of crap. She was above- she was superior. It's more likely that than your WW being scared of the evaluation.
I got my mother to admit once, only once, that she "might" need some help- but she never got it.
