I'm having a similar problem in Peahat World that I did in the monastery. I've gotten a second key and can't find anywhere to use it. I used the first key and took that all the way through Basement 3 and the maze and just hit a dead end (I did lower some blocks there). I've done everything I can see in basements 1 and 2 until I hit a place where someone tells me "this looks like a dead end."
Just going in circles now, not finding anything new to do.