Very nicely done! I really liked your interpretation of that prompt. I also laughed out loud at the little time-travel diagram Hermione drew for Ron and Harry. (Strangely enough, I was just thinking about the Patronus/multiple Harrys scene the other day. Probably a thought spurred by a future version of myself, egging me on to read this story!) Lovely!
I'll let you abuse my physics anytime!