The book wife required they actually put some plot into Robb's story
What story? Robb is that guy we hear other people talking about. He's very important in terms of plot... for the same reasons other major political figures are. We don't get a single scene of Robb and Jeyne alone, and if memory serves we get very few scenes where Jeyne even talks (and that, iirc, to Catelyn).
Robb's story in the TV series however is about Robb. Whether or not it was executed well is a whole other discussion, obviously.
The book wife is basically a one-scene wonder, and it would even more bizarre for an 18 year old to get into the situation with Jeyne that Robb did in the book.
Pretty much. If there's any reason to make her from Volantis, though, I've yet to hear it. If there are any further twists the character is supposed to have though I guess season three is the time to hear it.