I'm not sure what you mean with your "U" and "O" tracks, but I'd point out that the limitation on precision is imposed by the size of the probes. The size is limited by the fact that accidents could cause injury if the probes are very small, even if the pressure is quite limited.

Optical means can measure surface contours under zero pressure quite accurately, but don't do so well once some deflection occurs.

It's quite important to know not only where the hard areas are, but also how soft the soft areas are, as when they're very soft, they move about quite readily, and that affects the relative accuracy of foot placement, in the case of a below-knee leg prosthesis. If the soft tissue is allowed to move about too freely, one can't rely on where the footfall will occur. A very small movement within the socket can have quite large impact on relative position of the heel.

One other limitation that has to be considered is that the process is not instantaneous. Dimensional changes in the stump due to swelling, which can be quite rapid, has to be taken into consideration, so getting it done quickly is quite essential.

When I said the spacing of the probes it to be on the order of 2 mm, I donsidered that an absolute minimum. I'm persuaded, first by manufacturing constraints, and secondly by limitations on incremental benefit of proximity beyond half-a-centimeter, that it should be in the region of 50 mm. For a proof of concept, I'd even consider 1 cm, which is on the somewhat more than 3/8-inch.

I would not consider moving the probe set while pressure is applied, due to the risk of tearing skin.

I'm not sure whether it's more important to measure the probes' deflection concurrently or one-at-a-time. I'm also not sure whether it's more beneficial to measure the probes on a horizontal plane or a vertical one. Both have potential benefit. I'm not even certain that it would be better to use one probe or many.

RE