What happens when my empowered Gangplank, Naval is moved by my opponent’s Blast Cone’s play effect and they exhaust Blast Cone to stun Gangplank? Will he be stunned? What happens if I then play Back Off choosing my Gangplank? Will he get +3 [M] this turn?

Blast Cone will stun Gangplank. If you play Back Off choosing your stunned Gangplank, he will not receive +3 [M] this turn.

Blast Cone’s triggered ability that stuns units doesn’t choose them, so Gangplank’s replacement effect can’t apply to that event. Back Off played on a stunned Gangplank does not actually perform the stun game action, because Gangplank is already stunned. It’s legal to play Back Off, but the instruction will be negated. In order for Gangplank’s replacement effect to replace an event of him being stunned, the stun game action must actually be performed. Similarly, if an effect that said “Friendly units can’t be stunned” were active with a friendly empowered Gangplank, Naval on the board, a stun effect that is played choosing Gangplank wouldn’t generate an event that Gangplank could replace, because no stun game action is performed.