I for one am gladyou did Riddle in in a way other than chop him up or blow him up or drill him with a .50 cal muggle bullet. Not that any of those are bad, mind you, but yours is different. And that is good.
My complaint is your proposal scene with Neville and Hannah. Hannah HAS met Nev's Grandmother back in the Hospital Wing in Chapter 16, and the plot break is significant. Now if she said something like "I haven't really gotten to know your Grandmother...." or something, that would work much better.
As for using Magic against Riddle, what does it matter if its a forgotten Oath or a Reducto to the chest that does him in?
Murphy's 20th Military Law: If it's stupid, but it works, it ain't stupid. ...
Author's response
- I thought up that ending about a year ago and wrote the fic to use it...
- Yeah, I boogered up the meeting Grandma thing. It's fixed now.