MTR, great theory. I don't have any definite answers, but I do have a few observations to share.
I would not put it past the Greatjon's brothers to try and turn the tables on Ramsay, but are they that clever to wait until they control Moat Cailin to murder Ramsay? I think Ramsay may trick them into being the first wave of attackers on Moat Cailin, thus thinning their numbers.
My biggest issue with this theory is the timeline. With ADWD supposedly happening concurrently with AFFC, I'd expect more rumors from the north, ie. the Manderly's dealings with Davos.
Bolton and fake Arya left KL before Jaime. How long before? I dunno. But it might have been long enough for word of Ramsay's defeat of the Ironmen to have filtered back to the Riverlands where Jaime presently commands.
If Brynden is to rally the North, he's had very little time to do it so far. The Reeds have had more time... and I think they are accustomed to secrecy.
As for Theon's just killing of Ramsay... well, the Starks did not get their hands on Joffrey, Jaime did not get to personally chop Vargo limb from limb, nor has Victarion done for Euron. Only Stoneheart seems to be able to pull off some measure of personal vengeance, but she'll never reach Lord Lothar himself. Martin may not allow for personal vengeance in his writing, but fate, divind providence, justice, an eye for an eye, reaping what you sow, or whatever you call it seems to be a part of AGOT. Now I know that many "innocents" have suffered and died also, but do we really know their stories? Perhaps the sins of their fathers have caught up to them, eg. The Patriot.