Sorry it's taken me so long to reply. I feel lately like one of those Trelawney types who can't even get out of the house without forgetting where I put my keys, then finding them and not being able to locate my coat, dropping the keys while excavating through a pile of clothes for said coat, remembering that I packed a lunch to take to work, mislaying everything by the time I've put my mental checklist in order, and just generally turning in circles while standing in one place even though all I'm trying to do is leave the house. The aura of chaos infects the simplest effort at the moment.
Anyway, I'm both grateful and deeply dismayed that you spotted this canon blooper. Dismayed only because I feel like an idiot. I know where the ring/locket mistake came from - it was a brain blip related to my raised-eyebrow conviction that JKR borrowed from Tolkien - but complacency's at fault this time: I'd mentally rewritten canon and didn't even realize it. It really does throw a monkey wrench into the works, doesn't it? This is what comes of not questioning one's imperfect reading, because it never even occurred to me that Grindelwald died after Dumbledore. The idea had lodged comfortably in my head that he died the year DD did.
It will be some time before I can address this, as I'm so terribly far behind with promised fics that are driving me mad through their malicious refusal to cooperate (although of course it's my own muse that's the problem here) and a massive beta I owe a dear friend. I'll have to brood over the solution, because of course, curse it, Gellert's presence is woven into scenes that, as you say, are supposed to mirror canon's timeline. There's no quick fix. I'll have to comb back through the scenes and match them carefully to DH and then ponder what to do. But probably not until my backlog's cleared a bit.
Thank you so much for speaking up. I'd much rather have readers point out my own shortsightedness so I can correct it if possible. If this were totally AU and I hadn't set up the fic as a dialogue with canon, this wouldn't necessarily be an issue. But in this case I want to be as scrupulous as I can. I'm actually surprised that no one's pointed it out until now.
So again, thank you, and please know that if the error sits there for a while it's not because I'm ignoring your suggestion; it's just that I'm on creative overload right now and already hyperventilating at my own failure to get my fictional house in order.