You wrote: "Since they immediately detained these suspicious characters, presumably whatever was contained on their body, in their instrument cases, or in the McDonalds bag would have been recovered..."
My very first post and I'm using it to disagree with you :) Assuming it wasn't a dry run, who says they still had anything on their persons by the time the plane landed and they were searched? What if they each carried a liquid component of an explosive or bomb? The plan is for each guy in succession to stroll into the lavatory, pour his component down the sink or in the toilet, and then all the last guy has to do is go in and drop a lighter or match? Maybe the 'no' signal the guy gave was to call off that last guy. Far-fetched, I know, particularly since I know nothing about explosives or aircraft plumbing, but that's the way I'd do it. I'd be curious to know if any of those guys had matches or lighters on them.
Good point. Whether any chemicals poured down the sink or toilet could be detected would hinge on their plumbing system and the chemicals used. Since the plumbing system is self contained, I would think forensic chemists could still be able to detect traces of suspicious substances, although that would be a most unpleasant job.