When you first entered the citadel and killed the guard, he dropped some citadel guard armour, which the DK suggested you wear as a disguise. Put the armour on and most people will assume you are a new recruit. You can always remove it at the end of the citadel, and go back and kill everyone after completing their quests and learning any skills they can teach that you want to learn.
If you are sneaking to avoid the DK patrols, they are only in the corridors the DK warns you about. Also, they were weakened quite a bit from what was originally planned, so you shouldn't have too much trouble killing them.
There are 4 corner rooms in the drill grounds, 2 north and 2 south (ring any bells?), each with a lever. When pulled, each lever 'says' a word, which form a sentence in the proper order. <span class='standouttext'>Spoiler : </span><span class='spoiler'>Pull the levers NW, SW, NE then SE.</span> (highlight the text to read)