I believe that when one player gets a kill, he gets the full score and everyone else gets half of the points added to their individual totals. I remember I used to get about 3600 points in RI as a lone bomber and the other guy, who was only going after fighters, got 1800 something.
The points specified in goals and events are supposed to awarded equally to everyone, but IIRC in retail FS2 they were ignored by the game altogether in multiplayer. Not sure if this has changed with FSO.
The difficulty does affect the scoring quite significantly, although I'm not sure what the exact proportions are. That's why everyone used to play on insane back in the PXO days.
Speaking of this issue, there needs to be a way to check your points easily in singleplayer. The points don't appear anywhere on the pilot stat screens for some reason.