"Activating" the dummy actors?

Looks like the only place I'll be able to use this trick is during the Floating Continent sequence when Gestahl is shown in battle, it seems he takes the place of Kamog since that name is available during this battle event. So in theory I should be able to change his character name before the event runs, unless I'm mistaken.

I'm not sure where you got this info but I don't think it is correct. Gestahl is an NPC ($1C) in the event and no properties can be set to a NPC like a name (see $CAE075 in event dump and around). For the battle event, Gestahl name is hardcoded in the dialogues and for animation purpose Locke serve as a placeholder for Gestahl and Terra for Kefka:

Code:

D0C844: 03 01 DF C9   Animation for character 1(LOCKE) at $D0C9DF
D0C848: 0F            Excecute animations
D0C849: 01 D9         Display long battle dialogue $D9 at bottom of the screen
                         "GESTAHL: Kefka! Are you nuts?!"