I had always thought that the anti-muggle coalition might be based on something sturdier than simple intolerance and prejudice. Voldemort, considering where he grew up, although he was a cruel and heartless bastard, might have had his own reasons to despise muggles. I would also include Dumbledore here as well - I find it hard to believe he'd let Petunia get away with child negligence after what happened to his own sister and (2? or 3?) non-magical boys (where they child-aged, or teenagers?, btw?). This is another reason DH does not sit well with me. If it were up to Dumbledore, he would not be content to knock the Dursleys over the head comically with glasses full of mead for how they treated Harry - he'd probably go Caveman-with-Elder-Wand on them and transfigure-torture them a la Moody-with-Ferret!Draco.
This is heartbreaking, but thank you for writing magical!theory!fic. (will add to Memories) I am just glad that Snape got to Harry in time, before Petunia's ignorance resulted in another tragedy like the one with Ariana Dumbledore.