I don't use any soil as all. I just wrap the sphagnum moss tightly around the rock. Secure with fishing line. And then I place the plants on the ball and secure with more fishing line. The roots I have found will go into the sphagnum moss once they get going. And I jut lag the plants on top and secure with fishing line. After they start growing the stems will give shoot from the nodes along the stem and those shoots will grow upwards.
I tired your method and it still isn't working. Everything just rots away without soil. I think the moss was even dying, maybe the moisture not high enough.