We visit Kleve NRW regularly and take the ferry from Harwich to Hook of Holland. As Kleve is so near the Dutch border we often stop on the way to see friends in Nijmegen. One of them actually lives in Beek.
From what I have heard and read Marie was no friend of the Nazis and helped many Jewish refugees to escape on their flight from Germany. She would give them money and send them over the border to Holland (then free) with the help of her gamekeeper. She collected modern Expressionist art, the sort of "degenerate art" (entartete Kunst) that was frowned on by Hitler, and when HG or other Nazis visited her house she would hide the paintings and put up family photos, including of course pictures of HG. When she returned to the house after the war it had been plundered and the paintings had disappeared and have never been found, though another later artist reconstructed them from old black and white photos, so they hang again on the walls.
Other wartime stories tell that during Operation Market Garden when Canadian soldiers entered the house they came in through the windows and she sarcastically asked them whether it was usual to enter a house through the windows in their country. She heavily disapproved of the artist Joseph Beuys, who always wore a Homburg hat, supposedly to cover wartime injuries to his head, as she found this socially unacceptable.
You should visit Huis Wylerberg, Karen, though only the ground floor can now be viewed, as the upper floor is taken up by various nature conservation organisations. It's only open on Wednesdays and Sundays in the spring and summer. You have to park in the village and walk up the hill and volunteer guides will take you round. It now belongs to the Dutch government, having been sold to them by Marie's daughter.