Metallic mercury is not that toxic. In the 19th century, people even used to drink cups of metallic mercury as a laxative ...and they lived for a long time afterwards.
However, certain compounds of mercury are extremely toxic.
If you ingest even micrograms of one of these toxic mercury compounds, then your health (or even life) will be in serious jeopardy. Just read up on Wikipedia about the accident of Karen Wetterhahn.
dimethylmercury the most toxic of the series is over 200mg lethal dose, with obvious toxic symptoms at perhaps one 50th of that. Like methyl mercury it is a cumulative poison, methyl mercury causes Minamata disease.
The evidence seems clear that there is simply not enough mercury present in MDMA tablets to be an issue and most commercial MDMA and therefore most MDMA production by volume is not synthesised using aluminium almalgam reductive amination ergo most mdma will not contain significant amounts of mercury.
If the MDMA was made by reductive amination with aluminium amalgam with a completely incompetent synthesis then the mercury is going to be in the elemental form absorbed possibly on aluminium oxide hydroxide which is insoluble there is no realistic way that it is going to be in the form of methyl or dimethyl mercury where does the methyl come from?
The calculations are up thread.
LTC is more likely due to MDMA causing the surfacing of latent psychological issues in some people combined with the depressive after effect of MDMA itself.