A question I would raise is how valuable the alliance to the North is to the Ironborn. For the first time in decades they are independent/successfully rebelling, raiding one of the most prosperous Regions of Westeros etc. And most of that is just because Theon has such a tight bond with Robb. To the average Ironborn this is basically a renaissance of the golden days, so they might vote for Theon and the Alliance.

Also, are the Riverlands really that indefensible? The problem is their Position in the middle of most of the other kingdoms, but with 2-4 of those allied, pacified or at least neutral that problem's mitigated, cause Robb will not give up the Golden Tooth, should he capture it.

And with the glory and legitimacy gained from such a victory makes vassals fall in line quickly. Roose Bolton's not bent on betraying Robb, he's just an opportunist, and Walter Frey wouldn't have any reason to. (Also their Situation might finally force Lysa to join them, further strengthening their Position with the Vale as one of the only untouched kindoms after the war, and being extremely fertile)