Thank you! You're almost at the end!
I'm assuming the final key you need is the one in the entrance room. It's definitely the hardest!
Hint: There's a hidden path across the water that you'll need to make use of...
Another hint: There are three types of object you can destroy in the room. Could they relate to the clue you found on the signpost?
Bigger hint: You'll need to clear the room in a specific order!
Solution (ROT13): Svefg phg qbja nyy gur ohfurf gura obzo nyy gur ebpxf gura fubbg nyy gur pnpgv
(Just search ROT13 to decode the solution if needed!)