Those are all great points. I have a differing opinion, though, and I feel this is probably all it comes down to. And I would agree fan images work into it, but I don’t think that’s where this is coming from on my part. This is honestly how I felt about Snape’s interpretation in the book, because of the book, not because of seeing fans painting him like an angel.
I’m going by the feelings I had when I read the books way back when. I haven’t re-read them since, so the exact events are hazy to me. I remember hearing an interview like the one you mention, but the one I remember came before the release of the last book, which goes into the point I was trying to make, that, yes, the books did show Snape’s darker side, but only until it was revealed what side he was really on. I felt, in the text, as soon as we were told he was on the good side, he automatically became “good” by proxy, and he was shown as a perfect suffering angel. But, I think this definitely goes into people’s own interpretation of the text, which happens a lot. I didn’t feel, after it was revealed Snape was on the good side, that there was any more moral ambiguity presented on the part of JK. I’m not saying that’s what she definitely did, I wasn’t in her head and know it for a fact, just that that’s what her words read like to me. Especially given the actions her main character took at the end, and the final words he had to say about Snape, which had the author wanted to keep in a morally ambiguous light, I feel she could’ve very easily done. But she didn’t. Harry called Snape, “the bravest man I ever knew” and that was it. That doesn’t speak to any ambiguity surrounding Snape’s character. It is simply good. Had Harry said something more like, “This dude was kind of a huge ass, and a bit creepy, but he did some good things too, so I gave you his name” then I might agree that Snape’s character was still presented with that ambiguity.