If you want documentation about Eulenberg, that's q.e.d. by referring to the biograpghies on Wilhelm. I can aprovide three straight from my library if you want them. Yes, it is important because what it demosntrates in teh case of the Eulenberg scandal, Wilhelm's inability to judge the consequences of his actions and associations. It is not about his personal sexual preferences, although the press of the day had a field day over this, but about his ability to discern th econsequences of his thoughts, utterances, and deeds on Germany, and the rest of the world. In that aspect, it is an important pieces of the jig saw puzzles that provides clues about the causes of WWI, and the conduct of the war.
Historians are frequently in the position of reading other people's mail.
To ignore those facets of someone's personality that can be documented when doing biographies is to filter the portrait, and color the resulting image according to what we think is relevant. Sometimes, what they did is very important. In Wilhelm's case, the Eulenberg scandal is illustrative of his way of handling a crisis of his own making. You don't have to go far to find sources on this.