I'd take the bet that MDMA + talk therapy is more effective than MDMA alone, based on the theory of action that recollection is not a read-only operation in the brain, and traumatic memories remain as such in no small part due to the intense negative emotions experienced when recalling them. By forcing/encouraging recollection of traumatic memories, while subduing the negative emotions with MDMA, the connection between those strong negative emotions and the memories are weakened.

Basically, the MDMA subdues the negative emotions, and the talk therapy forces recollection. Without the latter, you just have a pleasant trip, but I highly doubt real progress would be made against the trauma.

It depends on how much study you do before hand and how much you go into the session with intention. If you're not good at self-education then I agree a therapist as a guide is helpful.

Worst-case scenario you might just have a wonderful time for several hours and decide you'd like to do it again with some kind of guide.

Anecdotally it's very uncommon to hear about people having a bad time or even a neutral experience on MDMA in any number of settings.