In the west of the village (just south west of the graveyard) an imp asks you to rescue her brother, who climbed down the pit to look for treasure. Following the trail of gems leads to the level 2 BF key.
In the lich cave in the mushroom forest, <span class='standouttext'>Spoiler : </span><span class='spoiler'>part of the northern wall of the southern east-west running corridor is not nearly as solid as it looks...</span>
Since you already have the level 6 key, finding the other 2 isn't necessary. The blocked areas are not related to the missing keys, or at least shouldn't be.
