I have no idea how to progress further.
I've looted all containers I could find, beat most monsters at least once (spider included), read books, mastered mana crystals, passed one of Focus trials (2nd seemes too hard at my level), I even used The Useless Key and went out to touch the trees!
But there's no sign of the third key to open remaining locked doors, no way through trees, no interaction on spider's green booty looty.
Sad.