Oh, wow, Author, this is amazing. This is just... wow, everything.
It's late, and I'm tired, and rather overwhelmed by how fantastic this story was, so apologies if I sound incoherent at all, but... yes.
This is so real. Everything about it, from the language and dialogue to the actual plot, is so Petunia. Grey, imperfect - there are bits missing in her understanding, in her viewpoint - the pace is detached even in the moments where things are unsettling - and the events are described in such a measured way - real - Vernon's attack(s) aren't tragedies but setbacks, the development with Frank and Alice isn't a triumph but a step; just like everything else in the story and her journey.
I love the supporting characters - Augusta and Arabella and Filius and Neville - all of them accepting but none of them quite understanding just how little of the wizarding world she actually comprehends. You capture them beautifully in the dialogue, especially - I'm quite sure I know who you are for reasons you will probably realise, but even if I didn't I think I'd pick it from the way you write dialogue.
The romance is perfect, and I don't think I've ever loved Percy this much before (honestly, I'm not even sure I've read a Percy fic before, though if he's as brilliant as this often then maybe I should start). They are quite similar in their temperaments, circumstances and views of the world, and the relationship is just so perfect and gentle and fragile (and Percy's talking at length about whether they are or are not involved and whether or not they should be is both hilarious and ridiculously sweet).
Lovely work.
One small thing you might want to look at/edit before crossposting after the reveal: when Augusta first mentions Frank and Alice to Petunia, she mentions DE's 'looking for your sister', which I noticed because it was pointed out to me in reference to one of my own fics eons ago that Frank and Alice were tortured after Lily and James were already dead. Later on, you've corrected it in saying that they were wanting to know what happened to Voldemort. I'm not sure if that was the 'canon failure' mentioned in your A/N, and that's the reason for the discrepancy, but just thought I'd mention.