their finger would hit the button if you killed them. death isn't instant
They don't die instantly, but they're immediately paralyzed from having their spinal cord cut and so they can't push a button to detonate a bomb.
cops are trained to go for center mass
When using a handgun and shooting to kill, cops will aim for center mass. But police snipers are expected to deliver a killshot to the back of the neck to make it impossible for their target to squeeze a detonator because the round from their rifle severs the spinal column before it is biologically feasible for them to react.
You'd drop what you had in your hands for sure if you were tasered. Unless you were wearing an aluminum foil hat before (to keep the CIA out of your noodle) and duct taped whatever you want to consider into your hand.
No, that's why they don't tase somebody they believe to be holding a bomb.