I can't believe no one has reviewed this wonderful story yet! And as your beta reader, I can't help but feel my work is being insulted as well! Well, I plan to put an end to that as well by being the first to review this story.
The first thing that struck me about this story was the believablity og Young Dumbledore's character. I had almost forgotten about his work with dragon's blood, but it was great to see that you remembered it. It's the smaller details of the books and when they are inculded in story that makes a big difference in how good a story is. In general, Dumbledore's character is exactly how I envisioned his character as a young man. We can see the curious nature that remains with him in old age, but we also see how it was possible for him to be influenced by Grindelwald, back before Dumbledore finally saw what kind of man he really was.
Reading this story also makes me think about your little Nazi boy story as well. I wonder how Dieter will come to view Dumbledore after he defeats the man who makes it possible for him to attend Durmstrang. But then I also think about how he will come to view Hilter after WWII is over and all of Germany learns the truth. Poor Dieter is going to have to suffer from a double-blow, losing two heros in his life. But, in truth, I suppose his reaction really won't be any different than any other German once the war ends and the blinders are taken off.
Grindelwald was terrific too. You have an uncanning ability to be able to see into the minds of history's darker individuals. It's almost frightening! But then, everyone has to have a motive, and I suppose writing Grindelwald's character of doing hat he did just because he's so evil is boring. It is also what makes you such a great author as well.
I only hope you can find the inspiration to write further!