Hmm. Not sure that adds up totally with Raith...unless he was the centrepiece of all their plans. Raith could be the anchor for all three Walkers. But I would assume he would be stronger. Clearly they also possess other bodies too and he shows no sign of being possessed by Before or Behind.
Sadly I put this down to inconsistencies in the writing itself as much as in-universe reasons. Jim isn't immune to the main problem of fantasy story telling: there is always a trade-off between consistency of established in-universe rules and conventions, and the conventions of writing itself. Yes, great writers seem to get it right more often but none of them get it right all the time, and not even most of the time. It's a very difficult thing.
The in-universe reasons you gave are as good as any. There is a bit of magic feather stuff with Harry.
Also just as an aside I meant Blood Rites not Death Masks before, totally forgot that book was in there.
I don't mean that he's possessed by all of them. I mean after spending millennia hoping mortals would summon them in ways that would give the Walkers the free rein to do what they need to do, they've suddenly got a willing partner in young Lord Rath.
Raith makes the bargain with Beside. In return, he's safer than any other Wamp, and with the knowledge and power at his disposal, he becomes King.
In return, all he had to do was allow Beside to bond part of his power to Raith (back-seat possession). Then, either with Raith's willing cooperation or by subtle unknown manipulation, Beside has Raith summon Before and Behind whenever they're banished or killed.
Suddenly the Walkers are a nightmare for the guardians of reality, who can never seem to find a way to get rid of them. They seem to be everywhere, and take on the appearance of an all-seeing, all-powerful Nemesis.
No-one ever suspects the White King, because he's a paragon of stability and order. He keeps things the way they've always been, and he doesn't rock the boat. Nemesis is known for causing chaos in its infiltration, and there's no chaos there.
Beside just rides his coattails, using him as a catspaw in the long game for centuries.
Right up until Maggie realizes his secret. Then it all falls apart. She threatens to out him, so he threatens Thomas. She agrees to keep his secret, but leaves. Then she feels a curse coming at her and suspects Raith, or wants to protect Thomas, so she uses her prepared death curse to geld him.
Now Raith can't feed, so he stops using magic that he can't replenish. He stops summoning Outsiders, and has to train mortals like DuMorne and Cowl and Madge how to do it instead.
Then Raith is made into a puppet, and there's no more summoning or training. But Beside still has him as an anchor to reality, and might have the ability to take control of him if need be. He doesn't, because once he does, the risk of discovery goes up. So he just bides his time and uses other puppets.
