Help with editing some in-battle events

You're looking for the Battle Event Script. There are a couple different versions available on the wiki. The script for when Sabin joins the fight is at D0/A49D. If you want the other 3 characters to stay in the battle you'll need to remove the commands from D0/A54E through D0/A567. You will probably also need to modify the animations scripts at D0/A633, D0/A5FD, and D0/A618 to stop them from getting blown off screen. That will take a bit of trial and error.