I will admit, I'm interested in seeing how you handle things with Neville. I could see Harry and Neville eating a meal together and, when Harry does the scan for potions et al., Neville's food/drink gives off the signatures, this could be just before or after Neville started to be affected. If it's after, Harry and company may already see the signs and suspect something (given how close Harry and Neville are, Hannah may come to the New Trio for help when Neville starts acting "funny" around Mandy). Really, Dumbles is stretching to try the same trick again under the nose of someone who knows about it. Methinks -this- is going to bite him like you wouldn't believe.
Oh, are Tom and Harry going to make contact at any point and discuss the "Dumbles Problem"? I'd reckon that could be an absolutely fascinating mental chat.
Author's response
Good guesses on the Neville front, but not quite...
Tom is going to start having his own problems. Harry is going to be in a pissy mood, and will be looking to take it out on someone...