Yeah, that last theory sounds about right. Edea never struck me as someone who would try to develop her own destructive powers, at least not of her own free will. Being possessed by Ultimecia must have given her some sort of boost in her magical capabilities. Not to mention the fact that, as far as I know, Edea didn't spend much time NOT possessed by Ultimecia. That is to say, I think she was possessed shortly after acquiring her powers, but it's been a while since I've played through FFVIII, so I can't be sure. If that is true, though, then it makes sense that she wouldn't have had access to the full extent of her powers because she had literally no time to hone her skills.

There is one hole in that theory, though, and it has to do with Rinoa. Remember the cutscene they showed us I believe a little ways into Disc 3? It took place immediately after the battle with Edea, and showed Rinoa possessed by Ultimecia. She revitalized a fallen Seifer and enlisted him as her knight, I believe. Now, as far as we know, Ultimecia can only possess sorceresses. What's more, Rinoa was shown in a similar state to her zombified mindset when in space. So we can pretty much be certain that Rinoa did, in fact, receive her powers right after Edea was defeated at the hands of Squall and Co.

However, we never really learn the specifics of sorceress magic. Perhaps a sorceress does retain a little bit of her magical powers after passing them down, which would explain Edea's basic attack and Limit Break. It doesn't sound like that much of a stretch, really.

If you look at all the sorceresses in the game that have powers I believe except for Adel they have a GF junctioned even Ultemica has the ultimate GF. Adel might be explained by being attached to Rinoa and absorbing her powers though. It would seem that sorceresses are just people with a heightened sense of magic as in they can recall the spells better, or maybe can fuse with the GF's better allowing them to tap into that power allowing them to use magic available only to the GF by drawing through the GF. Just a thought.

Actually, Ultimecia, Adel, and Edea were all without GF's. Ultimecia was only Junctioned to Griever during the third phase of the battle. Before that, Griever didn't even exist; she was actually the one who created him. Coincidentially, she created Griever with her incredibly powerful magic by pulling him from Squall's mind. So, basically, Griever didn't exist until after you defeat Ultimecia's human form. And, since she uses magic during that battle, we can safely assume that Ultimecia can cast her spells without having a GF. Junctioning Griever only increased her power. That doesn't mean she didn't have it to begin with.

As for Adel...I feel I must point out that Rinoa is not a Guardian Force. If memory serves, Adel was simply feeding off of her life force so that she could completely wake up.

I don't know if I remember her saying she was junctioning a GF or anything like that in the battle, or anything about her pulling Griever from squalls mind, but it has been awhile since I played the game. As for the Adel one I agreed that it was a hole in my thought process but it could be filled with the fact the sorceresses gained powers through the GF they had not necessarily still had. I sorta saw it as a process like I believe it was ff6 where once you junctioned them you could learn there powers than once you take them off after you have learned them you still have the power.

Thats why I mentioned Rinoa and Adel attached to one another she was taking in what she had learned to use it by going through her life force. They talk in the game about how they are losing their memories from the GF's even though Rinoa claimed she hadn't been using them as long as the other members of the party she was uneffected. Like I said the first time as well it was just a thought, nothing more.