You are still trying to make sense of the irrational.

For all we know, Petunia thinks "being magical" is a sliding scale, and if he doesn't go to Hogwarts to become even more magical, it will eventually fade. Or something. ("Or something" being literally part of her thought process.)

Also, in particular regarding to why she doesn't want him to go to Hogwarts, and about what she knew and didn't know, I think it would be supremely helpful to re-read PS. In Petunia's words, they did know he was "abnormal" because "how could [he] not be" with his parents; and they tried their best to prevent him from becoming like them -- and this includes preventing him from going to Hogwarts, for which he one day will be grateful.

In Dursley-logic, this makes perfect sense, but of course Dursley-logic is hilariously warped, which is the backdrop of the humour in early PS Heather talked about.

So actually, it's quite a clever bit of writing: It's delightfully absurd, but at the same time internally coherent, because the Dursleys as a whole are absurd, which prevents it from breaking the suspension of disbelief: Stuff like that is just what Dursleys* do.


*And actually, in a second layer, as a parody of us all: The quintessential Muggles, aspiring a house in the suburbs and a holiday home in Mallorca, stuck in a 9-5 job, narrowminded, without imagination, sense of wonder or ability to accept the different: Stuff like that is just what we do.