The difficulty spike: how would you address this situation?

Adding Mog is a good and fun idea, I went that route as well as adding Shadow so each party could have three people, and it really does help that whole battle a lot, allowing for the enemies to be tougher, or rather not basically forcing the player to rely on having an uber party of four. It is pretty straightforward event coding, a bit more than a couple bytes though. And depending on how you do it, you'd need to account for Mog's sprite possibly needing to do a pose like the other characters right after Banon says "They're coming!", if he is the leader of one of the parties. And a bit more challenging to have him there as a selectable sprite that you can change to and equip like the others, before the battle. I highly doubt you could fit this inline with the already existing event code, but if your ROM is expanded than this shouldn't be an issue to jump to free space.