Checkboard,
I'm not sure you've got that first bit right. If we assume that the engines are producing constant reverse thrust force throughout the manouvre ( yes I accept your second contention ) you have a constant negative acceleration.
So although the engines may not be producing as much total thrust at low speed, I would imagine the reversers would still be pretty effective. Some aircraft can reverse taxi using the reversers.
In fact, I recall being told that VC10 pilots used the reversers during taxi, and they had some failures due to the total reverse cycles thus being greater than the units were designed for. (I have no confirmation of this by the way, perhaps someone who knows can enlighten us)