and when this is done during an alpha prot pitchdown, you can observe the elevators smoothly go full AND
I doubt that.
From an academic point of view:
When alpha protect is triggered
- AP disconnect
- Nz law is replaced by Hi AoA Law
- Auto pitch trim is limited from entry point AoAprot. to 2° AND
- Elevator is 4° ahead AND
In Hi AoA Law AFT stick commands an AoA change from AoA prot (stick free) to AoA max (stick full aft).
In this case sensed AoA = AoA prot and - due to increasing Mach - AoAprot. ~ AoAmax (this is the referred Red over Amber on speed display), hence the lack of ANU control in AFT stick.
Now if the pitch wheels are cranked up (ANU) until level, this still doesn’t change the sensed AoA (remains blocked @ AoAprot) so there will be nothing to correct for by the automatics.
I would still wonder about the aircraft response if and when the vanes unstuck with full AND elevator and some considerable degree of ANU THS trim in place when this occurred.
Now that would be indeed very academic, but if this will be possible at all it would depend on other inputs. (Pitch, SS input, Airspeed)
Definitely it will be not a smooth transition into normal Nz law.