You do not need to kill Walker to get the key. See the screenshot instructions
here and
here. Also, in the latest game version, Nericon's garden gate can be lockpicked (level 5).
With loot dropped in scripted battles between the army and orcs, you can freely pick everything up after you agree to do a couple quests for the army. You can then also loot the armoury (at least until you complete the quests and are no longer considered to be working for the army).
