Mardino is a batshit crazy Wizard that you will encounter later in the game again (near the Slave + Master quest).
With high enough Perception you can find stuff even without other npcs giving you the hint as a questreward / buying the scrolls that reveal the location secrets. In this case Thuringer would have lead you to the buried treasure.
Not sure about the stag, I just kill every deer that crosses my path, because they have a chance to drop sinews for my archer.
