Much to my surprise, the build plate maintained its alignment during the thrashing around while installing the new X axis motor. Measuring the alignment with that routine gave these results in mm above the sub-platform:
Bearing in mind that the overall measurement accuracy isn’t any better than 0.05 mm (and I have serious reservations about that), the plate moves as perpendicular-ly to the nozzle as one could expect. I tweaked it a bit, but probably just rearranged the offsets.
So it’s stable. I must add a Z-height probe to the plate so that the machine can set itself up as the platform expands & contracts. The trick will be getting 0.1 mm accuracy at a constant offset from the upper build plate.