The therapist's disclosure to you in the context of joint counseling is different from the therapist's disclosure to a court in the context of custody proceedings. I'm skeptical that a treating therapist is going to want to get involved in a custody dispute. You cannot make a therapist testify as an expert (i.e., give opinion evidence), your wife has privacy rights that a court will respect, and if you cap that off with the therapist not wanting to be involved in the litigation you're going to be complicating your case by trying to force that approach.

What is more common in the context of a custody case is to ask the court to order a neutral psychological examination of the parties, in relation to parenting. The neutral expert can perform or review appropriate tests, interview the parties, potentially interview new partners who are involved with the parties, potentially interview the children, potentially prepare a report for the court, or testify at a hearing. It costs money, of course.