I think you're on to something. If you looked at the Potter books through special (magical?) glasses that reduced characters to their moral density the way orthopedic X-rays reduce limbs to bone mass, Harry and Voldemort would seem slight. Their lack of roundness, though, doesn't bother me as much as it does you. (I'm going to ignore your little caveat at the end, because you raise an important objection, and you leave me nothing to write about if you then dismiss it out of hand.) I think I'm more sanguine about their moral one-dimensionality in part because, as you say, the other characters complicate the picture to the point where (I think) they earn Rowling the right to glorify Harry and demonize his enemy. But mainly, I just don't agree that every hero has to have a dark side or that every archvillain must once have been a lovable boy. What if the insistence that every protagonist have failings and every scoundrel admirable qualities turned out to be the reigning cliché of our time?
Ambivalence is more seductive than its opposite, though, and I can see why you'd be tempted to demand that Voldemort have some. But now that I've finished the book--and I promise not to give the ending away--I suspect that Rowling is sketching the outlines of something even more perverse than the Miltonian backstory you propose. You want Voldemort to have become Satanic only after having been Luciferian--he was Head Boy at Hogwarts, after all--and to have gotten himself disinherited in some wrenching way. I agree that over the years Lord Voldemort has developed into a kind of vaudevillian Beezlebub, snakes and all. But there's a hint at the end of Harry Potter and the Goblet of Fire that Tom Riddle (Voldemort's boyhood name) is modeled not on Satan or even his latter-day equivalent, Darth Vader, but on a more troubling figure. Riddle is shaping up to be a version of KingLear's Edmund, a nasty piece of work who ranks with Iago and Macbeth as one of the great hero-villains of Shakespearean drama. Edmund, you may recall, is Gloucester's bitter illegitimate son. He betrays his brother and destroys his father, among other dastardly deeds, because, he says, the world has seized on his bastardy to deny him his due. No matter how eloquent Edmund's speeches may be, Shakespeare's audience would not have perceived Edmund's horribleness as a legitimate protest against his fate--against the arbitrariness of having your social status determined by your parents' marital relations, or lack thereof. Rather, the Elizabethans would have seen Edmund as a man at war with the universe since birth. Voldemort, the product of a similar background, has a similarly willful urge to punish the world for his misfortune:
You see that house upon the hillside...? My father lived there. My mother, a witch who lived here in this village, fell in love with him. But he abandoned her when she told him what she was ... He didn't like magic, my father ...
He left her and returned to his Muggle parents before I was even born ... and she died giving birth to me, leaving me to be raised in a Muggle orphanage ... but I vowed to find him ... I revenged myself upon him, that fool who gave me his name ... Tom Riddle ...
Comparing Voldemort to Edmund also offers us a different way to view what you object to as the dark wizard's "fire-breathing kitsch." Consider what Edmund does when the object of his designs, his brother, first walks on stage: He transforms himself into a stock melodramatic figure, all but rubbing his hands and twirling his evil mustache:
And pat he comes, like the catastrophe of the old comedy: my cue is villainous melancholy, with a sigh like Tom o'Bedlam.
In other words, Edmund may overplay his evilness, but that doesn't mean he isn't actually evil. He's both image and substance, hype and reality. He's a bad fake and a bad human being. He's bad through and through. Likewise, Voldemort goes over the top, but he's still malevolent. There isn't a sympathetic bone in his body. He's the dark force, pure and simple, his heart hardened against goodness because that's the way he is: nihilism in all its foul glory.
Likewise, I think it's fine that Harry is good. He's a hero, after all, and a young boy. Why can't heroes be heroes, the way they have been through the ages, particularly in fairy tales that deal with a young man's coming of age? Why must they all be modern and tormented? Although I do think he's less good than you say. I mean, he's no namby-pamby. He's got a temper, getting ticked off and playing tricks on Draco Malfoy. He often feels sorry for himself, particularly when it comes to his muggle relatives, the admittedly loathsome Dursleys. He's fairly insensitive when he quarrels with his best friend Ron, refusing to understand why Ron would be jealous at Harry's luck in being selected to compete in the Triwizard Tournament. Are these things meant to endear him to us? Well, yes, in part--but you're trying to have it both ways here. You insist on emotional nuance, then say when it's provided that it's offered in a cynical spirit. What could Harry do that would please you and be in keeping with his fundamental decency and stay within the confines of the books' light and charming mock-fairy-tale tone?
You're absolutely right about one thing, though: Dumbledore goes way too easy on the boy. I can't count the number of times I've wished Hogwarts would enforce its own rules, if only to keep Harry out of trouble. As you point out, the whole school and all good wizards everywhere benefit from having Harry wander off into Dark Forests where he gets kidnapped by giant spiders and other gruesome creatures, because in the end he saves the day. But think about it: Harry doesn't benefit from it, exactly. I mean, he lives and all, but he gets shaken up pretty badly. I think he's being used.
I assume you've finished the book by now. Ready for any grand summings-up?