In a handful of levels, in order to get to fastest time you have to lose points and get 0 stars. For these specific levels, for example Vos Cerveaux, Leave Fishies Alone, ECT there should be a different category for getting all 3 stars on it.

I think so. Besides, in the "Beat The Game" category, it's optimal to ignore the ruling, and get at least 0 stars in those levels. However in the "561 Star/100%" and the "285 Star" categories, be incredibly aware on how fast you're completing these levels. It is ideal in this scenario to be as close to the 3-Star threshold without dipping below it when the levels are done, but this is very risky if you don't know the scoring system to a T. As for the 'upcoming' "501 Star" category, I don't know what isn't worth 3-Starring?