You might want to check out the
NWN walkthrough. In
Blacklake the riddle, "a splinter delivered in flight," refers to an arrow. Stick the Ceremonial Arrow in the chest nearby to get past the door.
As for keys and doors, if you have the key needed for a door in your inventory, the door will open. The key is used automatically and stays in your inventory. If you don't have the right key, the door won't budge.