1. There can be (and usually are) more than one auction of pirates in a game.
2. When a line crosses the pirates' footprint the cargo has a chance to be taken, it's about 50/50 I believe.
3. No, pirates remain until they have stolen 200 resources, after that they leave. Sometimes people shut off their pirated resources, but this will only cause them to remain in position longer.
edit: nice x-post, glad we are saying the same