I also wouldn't be surprised if Marienburg had a stadholden as their ruler....
Indeed, they do.

As with the whole of the Warhammer World, there are going to be diverse influences on the background of Marienburg: the designers have always operated on the principle that they'll grab what looks cool and works within the setting and incorporate it regardless of its origin rather than sticking faithfully to one historical reference point (this is both a blessing and a curse of WHF background). The underlying basis for Marienburg, though, seems to me to be fairly obviously Dutch/Flemish, just as the Empire is a HRE analogue.