My question is, if the the MCAS starts to trim the nose down, can the AP be still engaged and save the situation?
I don’t fly the Max but if it was an NG it would be highly unlikely. First up, you want to be decreasing automation when things start getting away on you, not increasing it but if you did try for whatever reason, the NG wouldn’t take the AP if there was an active control column input from either side. From what I have been reading, the situation would be such that it is unlikely you could organise getting both crew members to release pressure completely while engaging the AP.
Anyway, if the solution to any problem was to engage the AP and watch it recover the aircraft I think many pilots would be contemplating a career change soonish.