Either you are going to feel silly, or I am going feel silly (in case this is a bug of some kind) but the final hand is just sitting there in the tower! When you solve the riddle, it opens up a room on the left hand side of the tower. That room contains the final hand - it's not a place to use the hand, it's where you get it!
Of course, if you posted this topic because can't pick up the hand for some reason, then that is a serious bug that I need to look into.
Also, thanks for playing (almost) to the end and thanks for the bug report about the items! I'll see if I can replicate and fix that.