"Escape to Salvation" - "Magnificent Retreat" [Campaign 4, Mission 4]

1) People usually answer these kinds of questions, but not me. I didn't have any problems here, so I can't say anything.
2) I think it refers to the city of dark elves. Just follow the storyline, you won't miss it.
3) See point 2.
4) Download the skill wheel diagram (for example, here) and you'll find out everything yourself.