The way you wrote that--Neville looking at the Diary but not being able to read most of it and the "flashbacks" was incredibly well done.  It was in my opinion, a rather sad story. I don't know if that was how you meant it, but it was what I took away from it (but not sad as in the dog died sad, more poignant sad).

I loved the His Twinkliness. That sooo needs to be added to the back of his wizard card (it's late I can't remember the 'official' name of them).

And I really liked your take on why Severus was giving detention.  It fits his character so well (not that he'd ever admit to it).

I also found the sex more as background scenery to set the stage for Minerva's story.  Heady perfumes and the frisson on fabric on skin trying to find fullfillment that is desperate but somehow enough.

I think the phrase "still waters run deep" perfectly applies to your Minerva. She had quite an extraordinary life for a "school-marm". I imagine Neville was quite shocked to read the diary and probably made a good choice to leave the letters alone.

I could only read a little bit of the story before it wandered into territory I'm not comfortable with - but I enjoyed the look into Neville's life - and really feel you showed great respect and admiration to Minerva by way of his obvious love for his former professor and boss.

I really admired the way you played out the perspective on this one -- Neville reading only fragments of the diary, while we, the readers, get to understand what is hidden beneath the cipher. The tone is also very special -- a sort of growing tension and dread beneath the escapism of the club. It's a story that really resonates.