True. I'm also inclined to not put all that much moral weight on some of it because of plotting concerns. JKR needed some reason for the confrontation to be put off until the end of the year, Sirius's transformation was key to it, and she'd put someone on campus who knew about it, so why didn't he tell? Um... convinced himself it wasn't true. Yeah. That'll do. And now, back to the story. It wasn't like she could address the issue earlier without Harry knowing about it--have Lupin satisfy himself that this wasn't the case, or go into his thoughts in any detail as he made his choices. Harry wouldn't have known about it, and that would have taken too long to explain during the climax in the Shrieking Shack. So when a major mistake is obviously necessary to the plot and the character is otherwise treated quite sympathetically, and the author says she'd want him teaching her children... I'm inclined to chalk quite a lot up to "plot device."