It isn't an exact science, nor a "hard" pitch count. Heck, even LL allows a kid to finish a batter after they reach their "limit." If the recommendation is 100 pitches, then 105 or 110 isn't that much of a difference. The idea is to prevent 120-150+.
As for catchers and other position players, there is a difference between pitching and throwing. Pitchers are throwing max velocity on every pitch. Catcher's do not. Trying to equate it to some percentage of a pitch is difficult at best. So while pitching too much can clearly cause problems, the question is, is there evidence that throwing too much is problematic? We have all heard the saying, kids don't throw enough.
IMO, the biggest thing with catching and pitching is fatigue which is really what leads to problems.
A catcher making throws down to 2nd base certainly is throwing max velocity.
Yes, but a good catcher only has to throw a couple of times before the opposition gets the point.