particularly if they had displayed a convincing looking device to the
crew?
Minor problem of determining what is a "convincing looking device"? Doubt they are marked Product of Islamic State' -- with a "best detonated by" date.
Even real ones may be made in someone's kitchen.
Who makes such determination, and how?
Invite member of flight crew to emerge from cockpit to inspect alleged bomb? (Is that task best performed by FO or Cptn?)