does the code in the Shadowlands also unlock the third mini-game?
Once you unlock two mini games, third third unlocks itself.
or stumble across fake walls that look just like real walls
No.
Is this passage worth investigating further? Or is it simply a decoration?
It's a decoration.
- On the message board I am hearing about something called "Hidden"... is that a bonus level on the map? If so, does the game hint at the way to find it, or would finding it on my own without hints be complete dumb luck?
Third secret game.