Yes, should be 5-HT. Sorry
To answer your question, when the MDMA is working, it gradually drains 5-HT out of vesicles. Then when the MDMA wears off, when 5-HT cells fire and the vesicles dock, but they don't release very much 5-HT. Normally this would be fixed by making more 5-HT, but tryptophan hydroxylase activity is also impaired. So the net effect is that extracellular 5-HT levels decrease.
It isn't necessarily the case that there is no serotonin blocking dopamine uptake. There certainly might be some. But it may not take all that much dopamine in serotonergic terminals to cause oxidative stress. So if more DA gets in than normal, into a terminal that is already stressed due to hyperthermia and oxidative stress due to MDMA metabolites, then that will contribute to degeneration.