I have a fair number of headscratchers about this game..

[LuckyBlackCatXIII]

First head scratcher: Since Wilhelm served Alexander from 1702-1704 (from what I remember from the "Remember" document) I think it's safe to say Daniel was not around at the time. I'd say Alexander did this around the last year of his services

Second head scratcher: It's heavily implied that Wilhelm and his men were transformed into monsters via Alexander's wine. Aside from the flash back and note that we find in the hidden study in the first library, there's also a loading screen that mentions this:

"The servants avoided him, yet he knew they were there. They all smelled
of spiced wine, a mystic scent permeated by clove and sage."

Also I think that FG mentioned in the commentary parts of the game that the gatherer's are basically bodies inhabited by beings from Alexander's world so...somehow he can control them I guess!

Hope that helps!