^Yeah, the breaking-up option has to be considered, though I think it has looked less likely since the Red Wedding.
In some ways, I think there would be a nicer symmetry if it turned out that Jon was Ned's son after all and some sort of alliance between him and Dany - which would be a meeting of Ice and Fire - was what saved the realm.
If Robb did sign a document that turned Jon Snow into Jon Stark, Jon would get what he always wanted. (Does something signed by a would-be dead king still count?) Ultimately, if Robb took steps to legitimize Jon, what would be important to Jon is finding out that Robb wanted to made him a Stark legally. I think that would mean the world to Jon.
I felt so bad for Jon when he was thinking on Stannis' offer. He remembered playing with Robb and shouting that he was "Lord of Wintefell." Robb said that Jon could never be the Lord of Winterfell because his lady mother said Jon was a bastard. (I really hated Cat for her loathing of Jon. She should have taken some of that ire out on her precious Ned - not a small child. I understood why she hated Jon, but still...)
I would laugh my backside off if the bastard of Winterfell defeats the others and marries the beautiful princess.