My ex made similar allegations -- that my SO was a pedophile. When it came up in court, my L stated the facts and the judge pretty much shrugged. Believing that your kids are being molested and then doing nothing about it, especially when there is no evidence (except for a history of high-conflict) has a boomerang effect on the accusing parent. It makes him look bad. "Why, if you were concerned that there was abuse, did you not report it?" And then he has to say, "I couldn't be sure. I had a sixth sense." Blah blah blah.
I agree with what catnap said. Your ex is looking for negative advocates. He suspects the modification of custody will go to trial and wants to have "witnesses" who can corroborate his story that your fiance is a threat to the kids in some way. Something that your fiance's BPDex may be happy to corroborate.
One thing about this:
The girls and I knew this wouldn't go well, but we did what a court and attorney would ask us to do first before requesting to modify.
My lawyer would never have asked me to have my son to do anything like this. Do you trust your lawyer? This might work for a low-conflict custody environment, but not high-conflict. It makes me wonder if your L understands what these cases are like. If not, that can seriously impair your chances for a good outcome.
It's different depending on where you live, and everyone ends up with a different judge, but in my case, the long, ranting emails from N/BPDx were very helpful. It was clear he was controlling, unreasonable, abusive, and frankly, unhinged. And what was even better is that N/BPDx would use these emails as his "evidence" that he then asked me to read aloud. So weird. I would sit on the stand and read these nutty and abusive emails and then my lawyer would stand up and say that N/BPDx had serious anger issues that made it impossible to coparent and we should terminate legal custody, and finally -- terminate visitation.
Sometimes those documents are the best thing you can hope for. Keep all of them. And don't respond in any way that comes across as negative engagement. You don't need to remind him that he is to blame -- let the court come to that conclusion on their own. Otherwise it looks like you are inflaming the situation and might have anger issues of your own. Either don't reply or answer, "Please confirm that you are not amenable to changing the custody schedule."
Sorry you're going through this. Incredible that he is a mental health professional.