the 1945 date for the original being destroyed actually makes me cautiously trustful, since that fits with what Klosterhuis says happened to the papers.

I was actually thinking the same thing, since you told us about the 1945 letter destruction. Supposedly (news article, slightly more trustworthy than tumblr) there's also a surviving 1729 (original??) of a painting *by* Hans Hermann, which I'm dying to see. I don't suppose it could possibly be the famous double portrait miniature of the royal siblings? W does say he had it with him almost right up to the end and took great care to ensure it was preserved, doesn't she?