He could have re-ratted to the Lannisters.

No clue. I’m utterly baffled by what they’re going to do with that botched storyline.

Eventually, yes. You need to build up population first.

Not really. The North has a population of 3.5-4 million; compare that to 100,000 in the whole of the Beyond-the-Wall.  

1. Probably the Byzantine strategy of evangelizing (both religious and cultural), bribery and divide and rule, mercenary work, and military conquest comes closest, but it’s still more of a delaying tactic, and was never that successful. 

2. Well, you’ve got the crannogmen who come close. But the reason why you don’t get them elsewhere is how those regions dealt with invasions – the Riverlands and Iron Islands got overrun way too fast with nowhere to run, the Westerlands, Reach, and Stormlands were strong enough to pull off something close to a Byzantine strategy where they combined intermarriage, conversion, military victory, and land distribution to integrate the Andals into the First Men power structure, and in Dorne, the combination of intermarriage and greater localism meant that First Men power structures survived in the west.

3. Because GRRM wanted mountain barbarians to threaten Cat and Tyrion, and then had to figure out where they came from, way back when his worldbuilding was still in flux. 

Ethnic and religious issues, primarily. The Vale clans are First Men, the men who took their land are Andals; the Vale clans follow the Old Gods, the Andals follow the Seven. So there’s a really sharp cultural division that has hardened over thousands of years, and a real sense of alienation. 

Compare that to the hill clans of the North. Rather than a subjugated minority, they were brought into the realm by the Starks defeating the Flint Kings of Breakstone Hill, the same as the Umbers, Glovers, Ryders, Slates, Lockes, Reeds, etc. Rather than being isolated, they’re part of a pattern.

Add to that that the Starks have worked to integrate them – the hill clans trade their cattle and shelter for the winter in the shadow of Winterfell, the Lords of Winterfell come in person to ask for their support in war, and they’ve intermarried with the hill clans. Ned Stark’s kids have Norrey blood in their veins, and their great-grandmother was a Flint. 

Maybe? Down the line? But at least in the early stages, building up Stony Sept is easier and is more strategically important. 

It’s not a terrible idea, but there’s an underlying problem of thousands of years of guerrilla war and vicious reprisal, as well as the fact that the lords of the Vale are occupying land the mountain clans believe belongs to them.  It’s an imperialist/colonialist setup, and there aren’t many historical examples of that ending well. 

You might be able to divide and rule, set some of the mountain clans against the rest, and then forcibly relocate the mountain clans down into the flatlands to be put to work and assimilated into the dominant culture – essentially following the Welsh dynamic already extant in the Vale. But even then, you’re talking about colonial warfare and the attempted obliteration of a culture.