Good points. My take on it is actually, all of the above.
The resoning behind that is, unless the charter is written by a part that is much stronger than the rest (and who can afford a "Take it or leave it" approach), it probably is a compromise by what the original members want. As the original members of UFP most likely was on equal footing, it probably is riddled by compromises.
Earth, barerly climbed out from the Post WWIII darkness before thrown into the war with the Romulans, probably wanted a strong defense force in case a new war erupts (but to war weary to have any true hawks in the top, so no one is interrested in conquest/offensiv expansion)
Other members, as the Vulcans, is much more passifistic and would probably not like a military union. A defence alians would be enough from that perspective. But exchange of knowledge and joining resourses for exploration would probably be high on their wish-list.
When war fresh in memory, diplomacy is usually prefered. So UFP might originaly be a defense alliance with knowledge/cultural exchange and trade agreement to strenghten the ties. Starfleet ends up as a paramilitary organisation with the primary mission to search for new life-forms, creating diplomatic ties if possible to defuse any future incidents (and if that is not possible, to keep track of potential enemies). Ad a century or two and both UFP and Starfleet had morphed, but the original ideas are still there at the core.