I can't answer all of your questions, but note that standard M5 threaded rod has a pitch of 0.8mm, not 1.5mm. Printers will often also use microstepping, which increases the theoretical resolution even more. Various other factors come into play, however. For example, backlash in the threaded rod drive will create a positioning error. Also note that even though you might be able to mechanically position to an ultra small resolution, you also have to look at what the firmware and the extruder hardware have to do in order to extrude the right amount of filament for that layer height. The printers I'm aware of don't have as much resolution in the X and Y axis, so is there much to be gained by having an ultra fine layer height, especially when it increases the print time?