How is she going to avoid losing her mental stability if she doesn't have Sansa and Arya back by then? She'll still have a crippled Bran to take care of and Rickos is too young to help with anything in a significant way. A middle aged woman really can't ride into battle herself wielding a sword in revenge. How will Catelyn even deal with it?
Plus, with Jamie, Joffrey and Tywin firmly on the "evil" side of the register we need a little more balance in the major male, non "evil" characters. Tyrion can't do it by himself. They're going to have to shore up Jon Snow's story. No way around it, but he's outside of the Starks' orbit north of the wall. I don't know how that's going to work. Perhaps Arya and Daenarys will get more focus. I haven't read the books, so I haven't got a clue.
Yeah, if TPTB play it right they can milk Robb's death for all it's worth since Madden makes him so sympathetic. At least we know they'll do it better than Berman and Brannon Braga did with Trip Tucker.
Hard to do it worse.