Heh. The problem with writing novellas is you have to wait for people to have time to read them before they can comment. :D
Yes, she was going to say humans. She may never really get good at automatically calling herself a Muggle. Heh.
And yeah, Vernon's canonical behavior is violent toward Harry, though he seems the type who would never attempt to get physical with anyone with any hope of defeating him (which of course is why he does hate Harry so), but Petunia, after years of bowing to his wishes? She might be a target, at least occasionally, and I can imagine him just getting more angry and frustrated and upset during their year away, as well.
I'm glad you noted Augusta's comment on the oddness of Muggle postal systems, because of course, for Petunia to see that her ways are just as foreign and confusing to an obviously clever and strong woman/witch, that's part of what helps her grow, too, you know?
Anyway, It occurred to me early on as I was thinking about this that Percy and Petunia had this common thread of feeling overshadowed by siblings who were too loud, too special, too much, so I figured that something in common would be something to build on, and if I could put Petunia in a position of some power or specialness, with the Alice thing, then there would be a reason for her to learn about this world.