1. you don't unless tested and if there is no reason to be tested you will not know.
2. Because it's not about the individual, it's about stopping the spread.
3. Not 100% in either case but the vaccine does grow the body's immune system and that should kill the virus but timescales are unclear. It is also unclear that people that have had it cannot get it again, a few have most have not and if they have their own body has dealt with it. eg the PM has had it twice, the second time was no where near as bad as the first.