Have plenty of friends that have overcome social anxiety they've had their entire lives with just one night of MDMA.
While I think MDMA-assisted psychotherapy is extremely promising, MDMA itself is definitely not risk free (especially when obtained from unregulated and adulterated sources); and various complications can and do occur.
I'll probably do it again. I just wish there were more studies so I could know how often is safe and optimal to take it.
(For the pedants: the positive test for the presence of MDMA does not prove that the substance isn't adulterated, but it's certainly better than a negative result.)
In my experience I had blue mondays way worse on my first 3 times, and then I've got to the point where I am no longer blue on the days after. That would be my 6 times over a 5 year period.
The 6 weeks number is more a rule of thumb. For some it's longer, for others shorter. In the beginning, it may not matter, but if you browse drug forums for a while, you'll see the pattern 'don't do it too often' emerge.
6 times over a 5 year period looks quite safe. I have some friends who have used MDMA more than that and some of them started to have longer 'sad' or 'depressed' periods, sometimes stretching into months. Again, it would be great if there was an authoritative manual for all of this, but I don't think something like that exists.