There’s no one answer to the hypotheticals you are putting. Law is made by judges applying the previous cases and the statute law to the particular facts as established by the evidence given in court. It depends on the facts and, even on established facts, different judges may disagree about how the law applies.
Not really. What a chickener-outer would need to do is definitely positively distance themselves from the agreed course of action. They’d have to leap up and say “I’m not doing this! This person should not be killed, and I’m not going to be a party to this madness! I’m going home and I suggest you all regain your senses and do the same. Bye-bye” and leave.
Remember that the conspiracy to commit (wilful) murder is an offence in and of itself. The minute everybody’s agreed what they’ll do and they’ve done something to make it happen (bought the gear to make the "shooting do-dad, bought bullets, whatever) you are liable for the conspiracy.
What these people have agreed is to kill somebody by using some elaborate “shooting device”. If everybody chickened out, nobody got shot, but they’d still be up for the criminal conspiracy. If the person got shot (ie the “right” button got pushed, and at least one participant pushed), it doesn’t matter if any individual claims not to have pushed, fronting up and participating would be enough. Having agreed that the person was to die by this elaborate method, and being there, you can’t say that you didn’t have a gun and didn’t shoot. That wasn’t the agreement. This ridiculous James Bond villain scenario was.
“The deed” isn’t shooting somebody. It’s turning up to a button-pressing event, having agreed to press buttons and kill somebody that way. Claiming you didn’t press doesn’t make you any less of a participant.
There are cases where a gang of people beat somebody up. If they acted in concert, you don’t get to say “I only hit lightly” or “I only hit once” to avoid conviction. If there’s evidence to support that you were only a “bit player”, that may be taken into account in sentencing, but it doesn’t get you out of conviction, if you agreed to go marauding around and beating people up. You’d have to speak up and LEAVE.
EDIT: firing squads and the like are not guilty of any offence by legislative exemption. As I understand it, they make it so no one person knows that they have fired the fatal bullet or pushed the syringe with the deadly poison for the psychological well-being of the executioners so they don’t have to dwell on it too much.