In my own sim trials the elevators were sufficient to initially achieve about a 10-deg ND pitch attitude, and the THS always began returning to normal when the stick was held full-ND.
Yup, that's what we saw too. I said it took about 5 to 8 seconds for the trim wheel to roll back to neutral after the stick was pushed ND (initially fully-forward, then maintaining about 10deg ND). The variance is based on the fact that I was counting the seconds rather than using a stopwatch (my hands being somewhat full at the time).
A manual setting of the THS was always available to the AF447 crew but it would have returned to the NU position once the manual rotation of the wheel stopped
As far as I know, there's nothing stopping a crew member from holding the wheel in position once rolled forward (and thus negating autotrim) until recovered.
Correct me if I'm wrong, but I think trim logic is set by stick demand, attitude and time, so as long as the stick was held forward while the trim wheel was rolled forward, I don't think you'd see a significant return to NU.