Response to the initial post>
Those are all wrong questions. It does not matter who rolls the Diplomacy check if he role-playes along with the check in a way consistent with his character. It might be fun for everybody. If the other characters are having problems with this, thay have to tell him IC. Unless they do so, everything is fine. The point is to have a game which is fun for everybody, not to play the best way possible.
Lacking skill points in a certain skills can not prevent you in trying that skill (minus some special skills). It is like you would be offended by a noicy character trying to make a stealth check or a character with low wisdom and no ranks in Perception rolling Perception check.
On the other hand, if you have a player who annoys other players (not a character annoying other characters - that is fine), than do not play with him next time or even kick him out during the play.
Unless Pathfinder Society rules say you have to play with jerks... ;-)
So, as you run things, the first (pushiest?) player to jump up and say "I use Diplomacy" is the one who makes the Diplomacy check? Anyone else can do no more than Aid him?After all there are no rules to determine who acts or which try will take precedence so it must just be the first to say anything, right?
Talking to someone is an in-character action. Making the Diplomacy roll is a metagame decision. I feel perfectly justified as a GM in listening to the -2 Diplomacy barbarian make a fool of himself and then asking the other characters what they're doing before calling for a Diplomacy roll. If they stand aside, the barbarian makes the real roll. If the bard steps in and smoothly covers the faux pas, he makes the real roll and the barbarian rolls to assist. Everyone who wants to gets to talk. Everyone who wants to gets to roll. Highest skill counts, the others assist.
If a player objects and demands to be the one making the roll, then that's an player level problem and comes close to the "Don't be a jerk" rule. There may be cases where characters are asking for different things with the social skills and then there may be in character reasons. One trying Intimidate and the other Diplomacy, for example. I'm still hesitant to always rule that the player who speaks first goes first, though.
Especially in PbP, where it's often just a matter of who's online when.