The trim setting will change in the FMS if you change either the derate or the TAss.
I've had the benefit of performing upto 4 take offs on the same aircraft, on the same day, at the same weight and the trim setting has been pretty spot on for each thrust setting (from full chat to max derate/assumed), with maybe a small blip or two either way - probably due to that inaccurate pointer or being too impatient for the gear to get up!
Not an engine failure but we do airborne engine slam acceleration checks one engine at a time and the stick force change between TOGA and idle is about 15 odd pounds (subjectively, I haven't used a force meter) which is not insignificant.
I've no idea about certification but I'd suggest that from my experience that the trim is valid only for the AEO case.
I've heard several explanations for the rotation pause - I'd go with ground effect personally but I also quite like elevator blanking by the wing. It's probably a combination of both and more.