I've spent the last little while, reading though all this. And I'm kinda torn. I have nothing against Canon characters, but at the same time, I agree that they can get confusing and complicated. BUT I do see some alternatives. If you want to have your charrie related to HP in someway...make something up. Something that is unknown, and don't make it a large part of your characters plot, but a small part. Then you've got a charrie that is involved in something we know about HP, but not something we ever had an answer to, but don't make it something that you broadcast

Though, I also don't have any objection to canon charries, as long as they are reasonable..and you develop your own plot and only use the cannon part as a simple fact.