I agree with you on many points, especially concerning the fact that us not having the facts makes coming up with the right answer impossible.

Though I would like to point out that Terra is referred to as the "Lingering Spirit" in the Japanese and to me at least, denoted its not so much the party facing Terra personally but just a fragment of his spirit or even a strong memory given form.

Regardless, the theory on Sora's connection to Ven is intriguing. I don't completely agree but it does seem quite plausible and I think most people have caught onto the connection of Terra, Ven and Aqua being connected to Riku, Sora, and Kairi. I am curious to see them explain the roles of the 7 Princesses of Hearts, Sora not being properly chosen by the Keyblade, the true nature of the Heartless and the Nobodies, and King Mickey's role in all this since the Birth by Sleep trailer hints he has been part of this whole mess from the beginning.