The key for Mardaneus should be in Lanilor's house after you've defeated Demona.
Dragon armor quest: Just the breastplate? Solve the Stone Axe quest and talk to the Dwarven King taking Axe and a certain letter with you. King will then tell you where you can get it.
The demon thing: Just a clue that there is a 2nd lvl in the mines - accessable with the Stone Axe quest.