I believe the grenade toss is necessary but it definitely can be done during the main story. I only used the guide when I was a little stuck and I stumbled over the pine cone/grenade by accident looking for the stone (hadn't figured out the word aligning

). Later on the bridge, "throw grenade" showed up. I struggled finding the right angle to make it turn yellow and since it wasn't mentioned in the guide as being relevant at that point, I abandoned it.
I've been walking around the rocky shore later but unless I somehow missed it, the scuba diver wasn't there.
Naturally, I wasn't too thrilled to find out I would have actually needed to throw the grenade after I finished the story.
I don't need to get a life. I'm a gamer. I have lots of lives!