the branches can lock in a way which the 3rd comb peice can't reach. I watched your youtube walkthrough, theres another configuration which diactivates the 2 larger peices and you're stuck after that.
reading the log when you save is like a story; kobold punished by the minion
the minion tore out a kobold heart
welcome to level 2 rogue
rogue picked up kobold heart
rogue picked up rune of ?
That's the hint button ;) Just try to do whatever the "phantom ball" shows you.