Thanks for the welcome and sharing your thoughts.
My underlying interest is just how accurate can a PLA object be printed or better perhaps is what is limiting print accuracy. Is SLA required for better print accuracy or can FDM be improved.
dc42 Point 1 is valid and it was not on my radar. I have a Cetus with a Z axis belt which I can coax it into showing micro stepping errors on selected printing tests. You picked a good printer geometry example. I thought about a springy leadscrew to motor shaft coupler supporting the build platform weight.
Point 2 I did consider but figured that internally the stepper is periodic every 4 steps or in my case 0.8mm and that one could choose a winding and polarity that resulted in some step position margin; however, I was thinking of belts and not screws. Your point 1 using a leadscrew might be hit and miss for Z, but probably not for the X and Y belts.
I found this link where they measured the stepper accuracy and it is obvious why thin Z layers have a problem with belts.
https://www.applied-motion.com/news/2015/10/stepper-motor-accuracy
Between two adjacent bad steps the difference error is 1/10 of a step which is 0.02mm for my belts. This is small until you think about the error as a percentage of a 0.05mm layer. Micro stepping errors would also subtlety vary the extruder nozzle flow rate.
deckingman. The interference fringe is sensitive to several printer errors but the trick would be separating out the individual variables and adjusting the printer. I posted an extrusion calibration test that you might like. It is based loosely on a micrometer. You print a nut and bolt and the snug final angular position measures the axial thread clearance which is known for the STL model printed. Flip the nut over and remeasure the angle and you get the overhanging thread sag.
https://www.thingiverse.com/thing:3390910