Although this is being posted a little late, I did accomplish my goal to complete at least 2 rocks in my mermaid's grotto. In fact, I managed four! Hurray! I'm surprised at how much difference these rocks have made. They're the first completed along the front right side of the grotto, and having them in really seems to pull the shape of the piece together.
On the down side, three of these rocks were ones where I didn't have that much choice of which threads to use. And making those choices for the other rocks has gotten me stuck for a good long while over the past several months. On the other hand, there are now only five rocks left, so I'm getting close to completing the grotto and moving on to the mermaid herself.