This is kinda the point of this topic, but if you like thinking that the story should've gone another way, or did something differently, that's totally fine. Knock yourself out.

However, know when you are in contradiction to canon. Again, there's no problem if you stand in contradiction to canon, but - and this is important - it makes little sense arguing against it. If word of god (as in the creators) state something as fact, whether you like it or not, it's canon. And arguing its legitimacy will lead to nothing. It makes little sense to participate in discussions where other people follow the canon if you yourself deny it. (Not that this should prevent you from denying it for your own enjoyment)

If you like the ambiguity in some parts of the original game, that's good for you. But standing in discussions with people who know how far the canon knowledge extends, there's not much point in bringing that up.

But well, that's what this topic is for after all - to discuss how we WOULD'VE liked the story to go. So let's get back on topic.

Personally, I liked the Brittenham theory when I first read it. Maybe just because it presented Sephiroth as more of a tragic figure than outright villain and I like that trope. But it made just as much sense to me when I later read that Sephiroth's will overpowered Jenova, and it made Sephiroth stand as more of an intimidating villain. So I don't quite know which I'd have preferred to be real. Probably the Jenova control theory, still.