This is an excellent story!
Fun, yet serious. And devious!
(It must have been hard for you to discard Parvati in favor of Padma ;-p)
I have to say that, the fact that you described the times Malfoy drank from his glass, hinted that there was something in it... And I did find myself wondering what it could be... A funny idea :-)
I really liked the fact that, in fact, Harry didn't even know this was happening!
Thanks for sharing!