If there had been the slightest hint, then I'd fully agree with the various points.
Never underestimate your enemy. The 9/11 hijackers didn't tip their hand about crashing into buildings even after they killed the crews. They assured the passengers they were returning to the airport (recorded on UA 93 when they keyed the ATC rather than the PA). It was only cell phone calls that informed those passengers that they were not following the prior hijack MO, prompting the passenger counter-attack.
Using one member of a hijack team, posing as a lone drunk or unstable individual, to frame the situation and flush out any unknown counter-measures would be a smart move before changing the plot. Prepare for the worse and hope for the best is simple prudence.