Dozy
What ran the trim up at the stall was the Airbus trying to effectively hold an attitude and being unable to do so without adding more THS trim.
Who really knows why the PF decided the nose needed to be 15 degrees up, but the aircraft was trying to make it happen.
If the nose had not been prolonged up by the addition of THS trim, it would have fallen through at the stall like most other aircraft.
In Normal law, additional nose up trim is disabled by angle of attack protection:
When angle of attack protection is active, THS is limited between setting at entry in
protection and 2° nose down (i.e. further nose up trim cannot be applied).
Similarly when the load factor is higher than 1.3 g, or when the bank angle gets outside
± 33°, the THS is limited between the actual setting and 2° nose down.
In Alternate law, it appears that all use of AOA is discarded. No doubt the engineers had their reasons, but I would be real interested in knowing what they were.
Instead, in Alt 1, you have Low Speed Stability which is derived from Airspeed indications:
Low speed stability
At low speed, a nose down demand is introduced in reference to IAS, instead of angle of
attack, and alternate law changes to direct law.
It is available, whatever the slats/flaps configuration, and it is active from about 5 knots up
to about 10 knots above the stall warning speed, depending on the aircraft's weight and
slats/flaps configuration.
A gentle progressive nose down signal is introduced, which tends to keep the speed from
falling below these values. The pilot can override this demand.
Bank angle compensation is provided.
In addition, audio stall warning (crickets + "STALL" synthetic voice message) is activated
at an appropriate margin from the stall condition.
The PFD speed scale is modified to show a black/red barber pole below the stall warning.
Va prot and Va max are replaced by Vsw (stall warning speed).
The a floor protection is inoperative.
In Alt 2, you have:
PROTECTIONS
Identical to protections in ALT 1, except that :
1. There is no bank angle protection in ALT 2 law.
R 2. In case of failure of 2 ADRs, there is no low speed stability.
On AF447, two of the 3 AOA sensors were functioning properly and 1 was lagging. The flight control logic was to disregard the outlier, so valid AOA data should have been available.
If the engineers had not decided to discard use of AOA in Alternate Law, AF447 might have been just a flight that scared the passengers a bit but no long term harm done. I will bet they are revisiting those decisions at the present date.