I couldn't reply to this earlier because I had things to say about the Dursleys that might have given away my own story. But yes, I went through the same process of considering whether Petunia might leave Vernon and deciding that, for now, she wouldn't.
She likes her middle-class life and its order and she wants it all back.
And this is precisely why. Just another example of how you hit my thoughts dead on with this story :).
And you're quite right, of course, that most of the characters don't really change, and that's partly why I love them - they're curmudgeonly and set in their ways! But they're also fundamentally nice (well, except for Vernon; perhaps well-intentioned would be a better word than nice), and it was lovely to see them all brought together here.