I always figured the real authentication was tied into the comm badges, and the authorization codes were just a fancy "are you sure"

Regarding the humanocentrism, there was one possible explanation I hit on while playing Star Trek Online and noticing the availability of species-specific ships (such as the Atrox carrier and the Andorian escorts) -- basically, most member worlds maintain their own independent fleets, but Earth doesn't. The human prominence in Starfleet is an artifact of that -- there's no official Earth Expeditionary Fleet, so the people who might join it apply to Starfleet instead.

The problem with pissing on Margaret Thatcher's grave is that eventually you run out of piss.