If Robb had successfully seceded, and had time to consolidate his Kingdom, as a Stark he would have to have his seat at Winterfell, but how would he exercise control over his vassals in the vulnerable “Southern Marches”, separated as they are by quite a bit of distance and the geographical barrier of the Neck? Is there a historical parallel? (obviously all of this requires some serious counterfactuals, like no White Walkers coming and a contested or weakened Iron Throne not coming after him)

Well, there are plenty of historical parallels when you look at the number of monarchs with geographically disparate kingdoms – the Kings of England owning provinces of France, the French owning Milan, the Hapsburgs trying to govern Spain, Burgundy, and Austria at the same time, etc. 

The success of any such venture ultimately comes down to a combination of good communication, delegation to local royal officials, the development of consensus-establishing bodies, and personal progresses by the king. 

Communication is, while not easy, somewhat easier than before when everything flows from King’s Landing. In addition to ravens, sailing from Seagard to Torrhen’s Square or from Saltpans or Maidenpool to White Harbor is a pretty short haul. Yes, Harroway’s Town to Moat Cailin is 700 miles, so you’re dealing with a two-week trip at the best to get from the heart of one kingdom to the border of another. But two weeks still allows for effective governance – the trip from London to Newcastle for example would have taken about 16 days at a moderate pace in the middle ages. 

You can already see signs of Robb developing the next two policies. Appointing Bryden Tully as the Warden of the Southern Marches means that he has a royal military governor in the Riverlands, but he makes sure that it’s a Riverlander and a kinsman so that there’s trust in both kingdoms. Likewise, having Edmure Tully swear his sword to Robb Stark means that the Lord Paramount of the Riverlands is formally a vassal to the King in the North – as with the Lords Paramount under the early Targaryen monarchs, the Lord Paramount is still going to be the supreme civilian leader and the 2nd most important military leader in the Riverlands as before, but Winterfell now has influence through future ceremonies of homage. 

Moreover, Robb’s policy of assembling the Lords of the North and the Riverlands together in council is a good one because (in addition to getting buy-in from his lords for royal policy) it gets the lords thinking of themselves as part of a single polity, and Kings of the North would be wise to hold mini-Great Councils on a regular basis. Separate assemblies are a bit trickier – the Great Council of the Riverlands could be a future basis for separatism – but that can be managed through regular appearances by the king to cement the feudal ties between him and his southern lords. 

Speaking of which, one of the ways in which monarchs project influence even within a kingdom is by making progresses and thus personally meeting and showing themselves to their subjects. If the Kings of the North want to hang onto the Riverlands, they and their kinsmen are going to need to make regular progresses in both kingdoms. It’s a lot of travel, but it does work. Progresses not only demonstrate respect for one’s subjects and inspire popular admiration through pomp and ceremony, but by offering an opportunity for local lords to lobby the monarch or settle grievances or judicial conflicts through adjudication of the monarch, it keeps local lords looking to the monarch for solutions.