I'm placing this in the ask box so this doesnt get lost in your motifs. What happens if Gift McGift gets sent into Jacob in New Moon? Rather specifically, when Jacob first shifts
@riote is referring to Gift McGift, Bella's sentient time traveling gift from the post-apocolyptic future who marries Bella in order to save her life. This happened on this blog.
Alright, rather than being its own thing, Gift McGift possesses Jacob instead.
... Is Jacob completely overwritten or does he now have a voice in his head? This makes a difference.
Jacob is Overwritten
We get a similarish timeline to the first, the difference being that rather than be a new, real, boy the gift finds itself impersonating someone who existed.
For Bella, this makes little to no difference. She's a depressed lump and if Jacob is acting odd then he's suddenly a werewolf. That probably explains most of the strange behavior (note that this happened in canon as well, as Jacob was suddenly much angrier than he'd ever been before, almost a different person, and Bella didn't blink.)
Jacob says he's Jacob and a werewolf now? Cool.
The issue is with the tribe.
Billy knows that Jake has now been through a lot, that he feels personally betrayed and angry at the world but--he's acting like a robot.
He's acting like a robot and it's almost as if he doesn't remember being Jacob, but like he watched Jacob's life on TV. He knows all the events, everything that happened, but he talks about it all impersonally.
Even things like his mother's death seem to mean nothing to him.
Sam and the other wolves note that Jacob's mind is bizarre. Being a shapeshifter barely seems to bother him at all and all he does is think about Bella Swan. He doesn't even pine after her, really, it's more that he clinically thinks about her and how to best protect her.
Suddenly, Jake's the one coming up with strategies on how they best kill the leeches and--somehow seems to know exactly who they are and how they'll work.
With his help, Laurent is quickly killed though Victoria is harder as she keeps running away (and will until lured in by the possibility of finally having revenge and the Volturi going to kill her moments later).
The difference here is that he desperately avoids Bella going to Italy at all costs.
Bella being turned would be great, but it's too risky. If she goes to Italy, she's just as likely to die by Edward doing something phenomenally stupid. He prevents her cliff diving experience to the best of his abilities and plans to kill Edward upon his inevitable return if he can.
Bella eventually moves on.
Jacob Has a Voice in His Head
Jake, at first, assumes this is a shapeshifter thing. That all of them have a weird, clinically detached, voice in their head telling them super weird shit.
Then Jake thinks it's some spiritual medium shit and that the voice is a wolf spirit or something that gives him unusually useful information on how to best kill the enemy leeches, what they're after, and even helps with his love life.
The rest of the tribe is dubious and not so sure but Jacob defends the voice in his head as being very cool and a badass. It also helps that the voice in his head seems to dislike Edward Cullen and only vaguely tolerates the rest of the Cullens.
Then it reveals that it wants to turn Bella into a vampire.
Jacob revolts, demands an exorcism, which probably doesn't work. From that point on, Jacob and the voice are on the rocks, and continue to be on the rocks when Edward does come back and Bella's suddenly becoming a vampire.
The voice just keeps nagging when Jacob now just wants to dump Bella because she'll never choose him or humanity. And then the voice is saying weird shit about Rosemary's baby and... what the fuck?
Then Rosemary's Baby happens.
Basically, Twilight happens but Jacob has an annoying voice in his head that is horribly, awfully, right at all times of the day. Edward just thinks Jacob is a little mad.
