I have read a lot ofdifferent threads about Jostero and the Harpooned! condition, and this got me wondering about dead man switch. Am I correct in understanding that if Jostero equipped with dead man's switch is destroyed, dead man's switch triggers at step 7, damaging all enemies at range 1. If one of those happens to be in his firing arc, he therefore gets a chance to trigger his ability and attack before being removed from play?

Dengar can do his revenge shot if destroyed, right? I'd be inclined to think Jostero could do this as well.

Yes and yes (assuming his timing is the same as qds).