I'd probably allow it with the human blood in there, just to drive home the "humans can breed with anything" that seems to be their superpower.
Humans can breed with anything. Dragons can breed with anything. Ergo, humans and dragons are the same!
Usually, my worlds follow the standard precept that orcs and elves cannot interbreed. In one case, I've used the Tolkien-esque conceit that elves and orc are actually the same creature - though nobody in that game asked to play a half-breed, so I didn't have to answer the question of what one would be like.
In this case, there's enough in the mix that the rules don't handle it well. My initial response might be, "Sure, you can do that, but the result is a character who gets no racial modifiers (good or bad), because there's no longer any strain in there dominant enough to impact traits."