Oh and here's an unsolved mystery to add to the list:

Who is Victor Sells?

Don’t know who he is, but as a character/plot device I think Victor Sells might be JB’s wink and a nod to Roger Zelazny. In RZ’s 6th book of the Amber series a fellow named Victor Melman was a low level wizard who was used as a puppet to try and kill off the book’s main character Merlin. Merlin was caught somewhat by surprise and ended up killing VM before he could get much useful information from him, thus leaving plot threads hanging and mysteries left unsolved for a while.

I see Victor Sells as a low level wizard who was a puppet, and who got killed by Harry before Harry could get much information from him. Had Harry known more about who trained VS earlier, it could have been helpful to Harry.