Not sure how it would look but what if you used your "shelf" pieces to create a bit of a "cascade" from one end and sort of ended at one of the 1/3 points. Then used the tonga branches to creates a negative space structure as the 2nd island towards the other 1/3 point.
Pretty sure that is clear as mud but in my head the types of rock you have screams mountain and forest. And you could get some neat structure with the tonga and create some cool coral placements. Hopefully one of the other members might explain what I am trying to say better!