[@Raddum]

Here's my thoughts, with all of this in mind:

Mateus explains his plot to mess with the summoning Cosmos performs, which is also used as his justification for why he can sense the heroes as well (he's basically connected to them by virtue of sabotaging the summoning -- this allows him to hide the fact that he was powerful enough to know where they were regardless). At this point, he'll be aware of Terra's existence, and for reasons I'm not revealing will immediately want to capture her. All it would've taken is Kefka mentioning her name or revealing that he knows her, and Mateus would convince him that they could turn her to their side (since you've already confirmed this would convince Kefka).

Assuming Kefka brings up Terra's name, it'd be a piece of cake for Kefka and Mateus to convince Zidane to capture her. I don't see any reason why Kefka wouldn't work with Mateus, since I'm guessing he can at least guess that Mateus is manipulating Marche (considering Kefka himself is a manipulative prick in the game, even if he's a bit less calculated and a bit more, well, insane).

I don't know much about the Crystal Bearers, but I'm guessing it's safe to assume that Layle and Jegran are an even enough match. This would make Terra/Ritz mostly having to deal with Zidane and whatever reinforcements Kefka/Mateus send, but I don't want to make assumptions about Jegran's actions until it's revealed which group he's attacking.

Things could actually get very ugly for Cecil/Freya depending on how Freya responds to something that's coming, but I've also got a trump card ready to help them out.