The ending (spoilers)
So at the end Christian's explanation, that Nevenka was possessed by Kurt and had killed his father and then herself, seems to be the plausible explanation for the happenings in the castle. However, during at least 2 of her visions (or hallucinations according to Christian) she has welts from being whipped, which would be quite hard to do to oneself though perhaps not impossible.
Near the end the director focuses the camera on Kurt's corpse burning in its coffin and the riding crop; suggesting he was buried with the whip. This introduced some doubt into my mind and makes it plausible that Kurt's ghost did return from the dead to terrorise the castle inhabitants and kill his father, and, that it was he who killed Nevenka at the end.
Anyway it's quite clever to leave an element of ambiguity allowing for interpretation either way.
my vessel is magnificent and large and huge-ishshare