Honestly, BD is my fav book, and Nessie was one of the big reasons. I could never picture Jacob w/ anybody else but 2 be apart of Bella's family...he was always meant to be apart of her family. I will admit, at one point, when Jacob broke from the tribe and Leah followed him because of her brother Seth following Jacob, and Leah and Jacob started getting to know eachother more, I thought maybe they're relationship would expand, but Jacob didn't really care for Leah much throughout the whole book. Nobody in the pack enjoyed her becoming a werewolf and Leah constantly would think of the whole Sam/Emily situation to make the pack feel bad, and Jacob couldn't stand that...yea, once they broke from the pack they started tlkin more and getting along, but Leah was still the same Leah, she still complained, and Jacob couldn't stand her complaining, and at first he didn't even like the idea of her following him, so I don't think Leah and Jacob would've ever worked out.
As far as Jacob imprinting on Nessie being weird, once u think about it, it's not all that weird afterall. True, she is an infant, but for how long? She's also half-vamp., so she doesn't grow like a human child would, i.e. instead of taking almost a yr to say 1 word, she starts kinda tlkin after like a wk or so. So how long truly would it take her to become full grown, or as old as she's gonna get because that was never mentioned. As far as Jacob's age, yes he's 17 (or however old he was @ the end), but as Jacob told Bella when Quin imprinted on a baby, when they're werewolves, they don't age...Jacob could stay 17 forever if he wanted to, and since he's gonna be around his sworn enemies the rest of his life, I doubt he's gonna age, so Jacob'll stay the same age, and Nessie will age for awhile, and at the rate Nessie ages, it won't take that long til Jacob and Nessie are if not the exact same age, pretty close to the same age.
And overall, Nessie is special, and Jacob being "prince" of his tribe is special too, so who would be better for eachother. And neither would have to keep secrets about their world to the one they love because they both live in that world. Plus, Nessie would be the most cared for, and most protected child, and in the end she will have a choice, she doesn't necessarily have to love him, Jacob will just be there for her, and if she chooses him, he'll be there.
"Look after my heart, I've left it with you."