Ok, there were 2 BF 1 keys - so you have only access to BF 2 because of the 3rd key. You're missing the 4th, giving access to 3.
Hiding guard => makes no difference, if you kill or not
Champion archer => you get more XP, more money and -1 rep for blackmailing him. Or you choose Rupert and get less XP, less money and +1 rep.
Secret entrance is beyond the exit door you have to unlock by using the levers (beware of the traps) in this order: NW, SW, NE, SE.
Kiya