Stables-Horse Farmer-Horse Breeder: Gives bonus to Cavalary troop types like Horsemen, Pictish Cavalary, Mounted Nobles, Royal Bodyguards, and Viking Riders.

Drinking Hall-Mead Hall-Royal Hall: Gives bonus to your factions Infantry troop type. Like Celtic Warriors, Gallowglasses, Highland Clansmen, etc...

Butss, Bowyer, Bowyer Workshop: Gives bonuses to Archers/Pictish Crossbowmen/Welsh Bandits.

Basically, if a unit comes from a chain of buildings that adds valour on upgrades to said building, they are the ones who get the bonus. At least thats how I think it all works, and its how i've seen things work in game.

Are you sure you had rebuilt everything? With the 3 types of forts before an actual Fort, things have gotten kinda confusing. Do you have a save showing where you can't build it?

Piety has become pretty useless, unless you're the Vikings. I don't see much of a reason to bother with relegion as a Catholic faction in the Viking campaign, just occasionaly sending Bishops to the Vikings to convert them.

Yes on this also, at least from what i've seen. I guess the unit they merge into carrys spare weapons/armor, makes sense more or less.