I think Jacob ended up with exactly who he was supposed to be with. I had posted once before the thought of Jacob falling in love with Bella because it was actually Nessie pulling him (even though she did not exist yet). There is magic in his blood, so yep, anything can happen.
While I was reading the books, i thought maybe he and Leah were gonna get together, and momentarily I was excited for Jacob because that meant he wouldn't be in pain anymore because of Bella. I didn't see him imprinting on Nessie until the moment it happened.
But Jacob is with who he was meant to be. He had to love Bella enough to risk his life for her so someday he would have his Nessie. Bella had to love Jacob enough to fight for Nessie to be.
So, Leah is all your for the taking! :)