Has a passenger or cabin crew member ever flown an airliner?
I think that in United Airlines Flight 232 Dennis E. Fitch did. While flying as a passenger on this flight, he actually was a certified flight instructor for the plane of that type so competent.

Chances to find a type-rated ATP between the passengers who would fly as if nothing may look slim but they are not zero. It is statistically more probable to find a pilot between passengers than would be just by random chance, because of deadheading.