Answer 1
These are all the maps that you can earn DP from by playing Zombies.
Pick and choose the map that’s easier. I haven’t played Zombies enough to know which one is the easiest. (I’d say The Ascension.)
Answer 2
Assuming you play 1-3 hours a day, I’d say a week or 2 at most if you constantly try to earn DP in those hours of playtime everyday.
Answer 3
I don’t know; however, I’d guess around Level 10-20.
You can always vote for the server to get loot boxes. If this doesn’t work, then there’s an error.