No, General Average is based on percentage of value, so the most you could have to pay is however much the stuff you were shipping was worth, if basically all the other cargo on board was lost except yours.
Yes, if that mythical container full of iPhones full of bitcoins got tossed first everybody else would be screwed. If that was common the next step might be clients with particularly cheap wares bribing the captain to sacrifice their containers first.
Absolutely! It's a highly theoretical scenario, but it would be that one time when refusing bribery would almost be borderline unethical (but if course deliberately sacrificing the value crate before others unless paid would be definitely unethical, and deeply so).