I've always said, give someone the psychedelics and a safe place to be, but let them be on their own. Lots of travelling to do, no need to stay tethered to this realm by having to converse with a therapist.
I don't see how MDMA can be useful for exploring relational issues. It's primarily a controller of emotion and mood, not a psychedelic.
So all it's going to do at best is make a client artificially imprint on the therapist - because the client would experience intense emotional closeness after it was administered, followed by a real downer after the session.
I'd consider that abusive by definition. It's certainly not going to have the much broader and risky but potentially more productive ego softening effect of real psychedelics, which would focus much less on the therapist and more on the client's interior world.
Sounds like you've not been around someone who is having a bad trip, it's traumatic. I've been around some and had to play caretaker till they came down.
A bad trip on MDMA? I've never heard anyone express that they had anything like the classic LSD "bad trip" on MDMA. It's just not especially psychedelic in that way, and broadly speaking, it's a stimulant that makes your body feel really intensely good.