Phew. I've finally finished reading all of these posts.
I've read a lot of great theories, and a few that I can't see happening. I don't really have a sure opinion, but I did want to mention something about imprinting.
In Eclipse, Jake says he doesn't think he is CAPABLE of imprinting. He says he only sees
Bella, and therefore is unlikely to imprint on anyone else. He may be around other girls, but he's not looking (poor boy!).
I believe the same is true for Leah. She only sees
Sam, and the pain he has caused her.
So when Stephenie says that imprinting happens the first time someone "sees" the other person after changing, we have to remember that "sees" can be a relative term.
My hope is that eventually (maybe in Breaking Dawn, maybe in other books) Jacob and Leah will heal enough to be open to imprinting. Maybe with each other (I'm not Stephenie, but she could make it work if she wanted to, so I'm not counting it out), or maybe with other characters.
One other thing I wanted to mention, is the last paragraph of the second page in "Being Jacob Black" (http://stepheniemeyer.com/pdf/nm_extras_jacob.pdf
Here Jacob mentions a girl having a crush on him, this could be our Helena. (If we do go with the Jacob as Demetrius theory, which I am in no means sure about)
I'm probably over analyzing that, but I had to mention the possibility.