I like this idea for a few reasons. For one, it removes pretty much all speculation from the books about Jeyne Westerling, and anytime a GRRM red herring/abandoned plot idea isn't completely sewed shut, I'm left still somehow talking myself into it coming back (I think what they did with Edric Storm/Gendry was great). For two, it definitely helps to really drive home Roose as a major player, which I still don't think the show has done...although, while my book knowledge is a bit dated, at what point in the books did it seem like Roose was really going to be a player, outside of a few throw away lines that don't really gain much ground until AFTER the RW?
The one part where I'm not wild is that I just think it's a touch too wrapped up in a pretty little made-for-tv bow. We'll see though. Unless they write a reason for her to have to leave the wedding at the last second (return home for a sick parent or something?) I see NO way she's either not a spy or just murdered...ending the Robb might have an heir storyline anyway.
As for the R+L = J "theory"...it's been so well speculated in the books now that people have cracked them open so thoroughly, I think the only way that doesn't end up happening is if GRRM decides to pull a Lost and change everything up since he's mad people guessed his big surprise. Part of me does kind of like idea of "Nope he's just a bastard, Ned really did it" is kind of awesome too, because I swear I know of no one that actually believes Ned fucking Stark nailed some lowborn woman and had a child out of wedlock, and just conveniently decided to never tell anyone, ever, who the mother was.