I have never heard anything about MCAS being remotely controllable, could be enabled or disabled inflight, etc etc. That sure sounds like a conspiracy theory.
What IS true is that the software has configurable features. For example, before these crashes, if you didn't buy the AOA gauge feature, you wouldn't get the AOA display in the MAX and if my memory is correct, would also not get the AOA mismatch warning even though the planes had more than one AOA gauge and used them for features like MCAS where you'd really want to know if the AOA gauges were in disagreement.
That's where Boeing is going to get roasted IMHO. This was an OPTION, even though the hardware was providing info to MCAS which was a required standard feature. And MCAS could be triggered inappropriately by an AOA disagree that wouldn't trigger a warning unless you bought the AOA gauge display feature. After the Lion air mishap, SWA got Boeing to turn on the AOA gauges, which pretty much involved just uploading a config file the next time the plane was at a maintenance base with the right software upload gadget and technicians. To my understanding, this will now be standard on all MAX aircraft since it sure appears that an AOA disagree *that was not displayed as a warning due to it being an optional additional cost feature* triggered MCAS, leading to 2 crashes.
Oops, talk about your all-time marketing foul-ups.