After reading through the posts so far, I'm loving the plans that are presented.
I'm personally leaning towards the Orb of Dragonkind (or a Dragonslaying artifact) option, since the PCs themselves could be sent out to reclaim the artifact. Thus, when they figure out the assassination plot, they will feel at least a little responsible. Hopefully.
Maybe have the Assassins use a ton of inhaled poisons as a backup, or even just to ensure the artifact has a chance to work, and it's a decent attempt. Potentially gives the artifact wielder a chance to escape should things most likely go south.
Since this is an Assassination attempt, and the villains would only be able to control the dragon temporarily with the Orb, it seems that coming up with a dragonslayer artifact is the best course of action.
Any cool ideas?