it would take longer, but be less complicated to just disable the PHS through the flevel then have the party be set to the desired party trhough flevel script. for scenes requiring specific actors in the party you would just need to have a party switch in the script at the start and again at the end. You could also code every character to stand in every event so the party selection could be skipped. but those are both 100+ hour projects that are really pointless since the game only has a few forced party member locations after you leave midgar, and for all of those only 1 person is required leaving you the choice of your 3rd.