Are you saying that the 737 cannot be controlled with the elevators if the stabiliser is the full AND position.
Does in the Sim at 250 kts.
By full nose down trim do you mean what you could get electrically? ie, around 3.9 units, or did you crank it all the way to zero? Did you also have the elevator feel cranked max nose down as it does when stall ID is reached?
MCAS is able to use a subset of the STS to run the trim electrically to zero units.
The stabiliser can easily overpower the elevator in my simulator experience, and evidently in actual life-or-death situations too.