MCAS was designed to only activate in extreme flight conditions (e.g. AoA being abnormally high). The reason why they decided have MCAS activate even without autopilot can probably be traced to this: https://en.wikipedia.org/wiki/Air_France_Flight_447.

The Airbus system certainly does override pilot commands. It doesn't matter if the autopilot is disengaged. There are situations even outside of normal law where the system will prevent the pilot from entering a stall.