1) I believe it's the killing blow. Credit for finishing the job, not for half-assing it. It's important to notice that we're listing the kills, but not ranking the players, as it's not a race to the top. Some classes are good at shooting stuff, other classes are good at falling to their deaths and getting to the drop pod first.
2) I think it's deposited, but I haven't actually checked. Should be easy to do. tho. Start a mission. Mine something, deposit some of it, fail, check the end result.
3) If you don't pick one you get one of the three choices at random (unless it's been changed, and no-one told me...). The reason it is like this is that we don't want people to reroll their choices by not picking an option (and in case you get disconnected, it's not lost).