So what is Your essential mechanism? Is it a coin toss?A method of resolving disagreements about what happens next. That’s it.
“I shot you!”
“No, you didn’t!”
Roll for it. Flip for it. Draw a card for it. Whatever.
Almost everything can be determined by the fiction. What does the fiction dictate? Do that. Not sure what the fiction dictates? Insert randomizer here. You don’t need much more than that.
I understand that you defined a situation, but You didn't specify which one of the mechanisms you mentioned is your preferred one.