After writing bits of info on "one" of the spells, it was interesting to test how it operates... the results are surprising.
If Older Rydia is not in your party, then the Golbez battle works strangely. The game freezes if there is no character in the middle slot. If there is a character, then they seem to do some strange movement in part of the battle as part of the "summoning Rydia" spell.. and the movement gave me an idea.
Older Rydia must be inserted prior to the battle. Her status/HP is not displayed until the "summoning Rydia" spell is executed. Once executed, Rydia simply comes out of the "Hiding" status (the same one Edward uses) and joins the battle. That is how this part of the battle works. I'm willing to bet you can change the character that appears by inserting them prior to battle. You can keep repeating this method with the FF2 Beastiary codes to properly understand this very battle.
We have EVEN MORE themes now, but some old ones are gone...