A) is very hard to test given the restriction on using magic around muggles. As for B), powerful spells are mostly restricted by the edict of Merlin. C) is, as you pointed out, extremely difficult to research effectively. I'm more surprised that Harry never bothered to ask how new charms are discovered. After all, how are you supposed to figure out that you are supposed to say "Wingardium Leviosa" and then move your wand in a certain way? And he as been told that new charms were discovered every year, so we know it's possible.