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.