A very interesting take on McGonagall, here. Her past flirtation with Riddle's magic and her guilt thereafter seems most plausible - especially as noone seemed to know/remember save herself.
I like the way you wrote Slughorn; this exchange captures him extremely well:
"Horace Slughorn, shut your mouth and do your duty," she said, "or I shall make you do it; you know that I can."
"Now, Minerva, you know that that isn't what I meant," he said, in wounded tones.
Just one tiny quibble, however: we know from canon that Slughorn was Tom Riddle's teacher, so he couldn't have also been Minerva's classmate. After adjusting, this didn't impact upon my enjoyment of your story, though. :-)
I also liked your Kingsley very much - I can see that he and Minerva could be compatible.