How come the pre-Andal Royces never absorbed Gulltown into their kingdom? It’s the largest port town in all the Vale and only a stone’s throw away from Runestone itself, so why would they leave it alone? It’s not like they couldn’t do it, the Royces are described as one of the most powerful forces in the Vale at that time.

They did try:

Farther south, the wealthy harbor town of Gulltown on the Bay of Crabs was ruled by Osgood Shett, Third of His Name, a grizzled old warrior who claimed the ancient, vainglorious title King of the True Men, a style that supposedly went back ten thousand years to the Dawn Age. Though Gulltown itself was seemingly secure behind its thick stone walls, King Osgood and his forebears had long been waging an intermittent war against the Bronze Kings of Runestone, a more powerful neighbor from a house as old and storied as their own. Yorwyck Royce, Sixth of That Name, had claimed the Runic Crown when his sire died in battle three years previous, and had proved to be a most redoubtable foe, defeating the Shetts in several battles and driving them back inside their town walls.

Unwisely, King Osgood turned to Andalos for help in recovering all he had lost. Thinking to avoid the fate of Shell and Brightstone, he sought to bind his allies to him with blood in place of gold; he gave his daughter in marriage to the Andal knight Gerold Grafton, took Ser Gerold’s eldest daughter for his own bride, and married a younger daughter to his son and heir. All the marriages were performed by septons, according to the rites of the Seven From Across the Sea. Shett even went so far as to convert to the Faith himself, swearing to build a great sept in Gulltown should the Seven grant him victory. Then he sallied forth with his Andal allies to meet the Bronze King.

King Osgood won his victory, as it happened, but he himself did not survive the battle, and afterward it was whispered amongst the Gulltowners and other First Men that it was Ser Gerold himself who struck him down. Upon his return to the town, the Andal warlord claimed his good-father’s crown for his own, dispossessing the younger Shett and confining him to his bedchamber until such time as he had gotten Ser Gerold’s daughter with child (after which the father vanishes from the pages of history).

When Gulltown rose against him, King Gerold put down the protests brutally, and soon the gutters of the town ran red with the blood of the First Men … and women and children as well. The dead were thrown in the bay to feed the crabs. In the years that followed, the rule of House Grafton remained uncontested, for (surprisingly) Ser Gerold proved a sage and clever ruler, and the town prospered greatly under him and his successors, growing to be the first and only city of the Vale.

If the Andals had waited a few more years, the Royces might well have captured Gulltown.

Is the Royces’ status as a principal house of the Vale at odds with their seeming pride in their First Men heritage? In a country noted for the value placed on noble Andal blood, whose First Men were mostly dispossessed and driven to become barbarians in the mountains, you’d expect any remaining First Men noble houses to try to assimilate into the Andal order – but things like ancient runic bronze armor seem to suggest the maintenance of a strong, persistent and distinctive First Men identity.

Actually, no, I don’t think it does.

I see the Royces as something of the Vale’s Loyal Opposition. They’re one of the most powerful Houses in the Vale other than the Arryns, they’re older than the Arryns (and Valemen are super-snobby so this counts), and no one challenges the Arryns except for them. Hence why a Royce cadet branch ended up at the Gates of the Moon, why we see Royces as Lords Regent of the Vale, why Royces marry into House Arryn (and I would imagine vice versa), why it was the Royce of Runestone who besieged Jonos Arryn when he turned traitor, and why Bronze Yohn was one of the first to turn against Littlefinger.

I see them as somewhat assimilating and somewhat accomodating. On the one hand, the Royces have converted to the Seven and are definitely bang on side with the whole idea of knighthood and tourneys and the like. So they’ve done the big ticket things that matter. On the other hand, they’re very proud of their First Men heritage – the runic bronze armor, the marriages into House Stark, etc. – but I think in a way that works for the Vale. Being arch-traditionalists, the Vale are very into heritage, and the Royces being Seven-worshipping knights means that the First Men stuff is made “safe.”

Why is Bronze Yohn so committed to the cause of House Stark to the point of being in “near open revolt” over Lysa’s failure to support Robb ?

The Starks and the Royces are kin: Beron Stark married Lorra Royce, so all Starks since Beron have Royce blood in them.
Ned and Yohn were contemporaries and knew each other pretty well: Ned was fostered at the Eyrie where the Lord/heir to Runestone would have frequently attended court, Yohn was at the Teourney of Harrenhal, Yohn almost certainly fought with Ned during Robert’s Rebellion, Ned hosted Yohn at Winterfell when Waymar Royce joined the Night’s Watch, etc.
Yohn rightly views the Lannisters as his personal enemies: Yohn was there when Robert died, he was put on Cersei’s enemies list, and managed to get out of the Capitol ahead of the Goldcloaks.
Yohn is likely one of those Valesmen who blame the Lannisters for the death of Jon Arryn.
Yohn’s a traditionalist and views Lysa’s failure to fight in defense of her own kinfolk as failing in the honor code of the Vale.