Don't want to throw a wrench into anything. Maybe just offer an observation. You mentioned "strong woman" and your fear of being assertive. Then you transition into, "should I ask permission". Taking your D to therapy when there are marital problems only sounds right to me. They get to talk to someone neutral. Why ask permission, why not just tell W that you think it's important and scheduled an appointment. Offer to let her take your D if she wants.
This might come of weird, but I think strong women can get put off by overly submissive behavior. I also think she will get mad at you when you stop. But I don't know if that's really a bad thing.
