You had me worried that Harry was going to be fooled! It's a nice glimpse of Harry and Draco together at the end, though the hard part is still ahead of them.

Severus' Curse Potion reminds me of 'The Red Shoes' where the wearer must dance to death. The impostor will run to exhaustion, then return to his death. A death which I am guessing will be as gruesome as Severus could make it.