How well defended is the passage to the Vale through Wickenden? Shouldnt the Waxleys be more important lords in the Vale because of the position of their seat in defending one of the entrances to the kingdom?

Looking at the Lands map, I’m not sure on reflection that there is a land path to the Vale through Wickenden, actually. 

See how the Mountains of the moon come down there to the sea behind Wickenden? If this is map is accurate (and it’s always an if with these things), then it may well be the case that there is no route there that an army could take, and that Wickenden’s links to Gulltown are purely naval in character.