Can the autopilot (or stall avoidance system?) of the B737 MAX 8 be overridden by sheer force?
I'm not sure of the full story here, but if you pull the controls of a 737 while the autopilot is engaged the autopilot will trip and the pilots can fly the plane manually. This requires about 25 lbs of force.

There is also a button on the yoke where you can disengage the AP. So if the AP was controlling the plane and going into a violent descent the pilots should have been able to cancel it by pulling the yoke or pushing the button on the yoke or the mode control panel below the windshield.

And no, the controls being hydraulic doesn't play a role in the "override by sheer force" possibility.

Edit:


  The autopilot servos are limited to 25 lb of force for single channel operation. Separate force sensors measure pilot column input and the autopilot will disconnect if the force applied by the pilot(s) exceeds 21 lbs.


Source