I think we're actually talking past each other, and since it was my rueful aside, let me try and clarify: I'm talking just about the characterization. Being a Silver Age character, and a female one at that, Gwen Stacy's personality was ... er, unstable ... and not only has she gotten a post-death canonization ala Uncle Ben or Barry Allen, but most of the writers who've tackled extended stories about her in the modern age still portray her as the Ideal Girlfriend. So not only are we supposed to see the Silver Age sloppy characterization as internally consistent, we're supposed to think it's the ideal of young womanhood. (This is especially creepy in Marvels, in my opinion.) Gwen Stacy is a character with no goals and almost no interests of her own, not because she's dead, but because no one thought about giving her any. Who cares? She's just the imaginary girlfriend.
On that level, "Sins Past" works for me as a valiant failure because it not only doesn't gloss past the inconsistencies, it brings the inconsistencies to the forefront -- suggesting that when Peter, the rest of the cast, and the reader were putting Gwen on a pedestal, she was snapping under the pressure. (Thus why I say the Gwen/Norman makes sense.) It showed Gwen with clear-cut goals of her own: modest ones, yes, but waaaay better than the, "The invading tanks are so pretty" of Marvels or the catfighting in Spider-Man: Blue. And by retconning the reason for her murder it worked really hard to make the Original Fridging a tragedy for her own sake, and not for the hero's. (Didn't really succeed, but it tried.)
Oh, and astonishingly enough it was a story about the girlfriend cheating on the guy which wasn't judgmental. Too bad fandom, uh, missed the point.
Of course, then there's the fact that the story, uh, sucked. And speaking on a meta-narrative level as you are, I don't disagree with you at all. I'm not sure I would draw as direct a line between Sins Past and Norman Sue, since I'm pretty sure Marvel Knights: Spider-Man ignores Sins Past, and Mark Millar wrote both MK:SM and Civil War -- but then, I also try and tune out a lot of what Quesada says these days. Apparently, uh ... it's for the best.
(It really weirded me out how blatantly Norman/Lily rehashed Sins Past and yet how hard they worked not to mention it.)
What irritates me about Sins Past is that there's an alternate universe story out there somewhere which would have had the same plot, but been written as a devastating indictment of the Silver Age Groupie's fascination with his fictitious, eternally virginal, Girlfriend-on-a-Pedestal. Goddamnit, that would have been an awesome story.