Was MCAS part of 737 8 MAX MMEL list?
It does not appear that MCAS was in any of the maintenance documents. This was part of the issue with the Lion Air crash. On the previous flight with the accident aircraft, MCAS acted on the faulty sensor data, but the pilots used the trim cutout and completed the flight. They had no reason to suspect that a trim issue could be related to a faulty AOA sensor, so just the warnings for IAS and ATL DISAGREE and FEEL DIFF PRESS were logged for maintenance. As you noted, if they had known the connection, they might have diagnosed the true cause and either replaced the AOA sensor or MMEL'ed it. Instead they focused on the Air Data Module and elevator feel computer. It wasn't until a couple weeks after this crash that Boeing sent a message to operators describing MCAS.

It's also unlikely that the MMEL would have allowed MCAS to be completely disabled. This would have required extra documentation and possibly training for the pilots on how the plane would handle differently without it. This additional training is what MCAS was designed to avoid, and would have defeated the purpose of having it in the first place. Also, if it were that simple, it seems like they would have taken this step rather than grounding the entire fleet.