It's more literally "just one sensor" than you say, I think. MCAS doesn't average or anything: it just picks one sensor at a time arbitrarily and uses it. (This will change in next month's software fix.)

(2) sounds like it could be airworthy, but I think Boeing might instead find that it has to add that third sensor. Airbus has three, and any rationale that Boeing only needs two because the disagreement consequences are not severe seems atop quicksand at this point.