It is common for threads on any forum to divert to another subject. This one because you are one of the few that has built his own machine. On the subject of differences on a square mast vs round, I prefer the round quick release type of my Facetron has vs the square crank type of the Ultra-tec I also own. If a square type mast is made correctly inherently it should be more accurate. You always see commercial milling machines with square mast for this reason. For commercial cutting speed is important to produce more stones so a quick release is preferred, at least by me. On a faceting machine I really do not see any difference in the accuracy of either one if they are well made.
Concave cutting? Not for me. No way do I have the patience for doing it but I like the results.
As far as diamond polishing, all my competition quartz stones were polished with diamond where I could read the grease and know what needs more work. Never had any points deducted for polish. For the cutting I now do I now use cerium and a matrix lap after a 3,000 prepolish on a batt. When I was commercial cutting years ago I polished and sold many amethyst, citrine and smokey quartz stones with a diamond 14,000 polish on a tin lap after cutting on a 600 grit lap. That was before the Batt lap which is a harder tin lap and better. This forum is mainly for competition cutting so it is not a technique that would be used. I have not found anything diamond will not polish if one uses the right technique.