Not sure about number 1, never faced that situation.
2- You are allowed to defend your gang members, people you are basing and/or raiding with. Nobody else.
3- I was told that 2nd door in PD can be AOS/KOS if there's a law specifying that you cant pass the 2nd door in PD.
4- You can't kill the owners of a base while they are outside unless you're sure it's their base (eg they just came out of it)