Looks like a great scape idea.
If I double the Red Clay in my Mix I don't think it would work in your application. I don't even think a Pc of Red Art clay used to make pottery would last very long under water.
1st thought is there anyway to hide a clay pot in the rock work to plant in?
If not, sounds like you might need to fabricate a bowl type/ retaining wall structure between the rocks to hold the MTS.
I'm not sure about Toxicity, there a product avail be at Ace Hardware called Vinyl-crete. It's premixed in a gt, cans. It's basically just what is sounds like Vinyl & Concrete . It sets up in mins. 14 min after set up you can drive a car over it. You could put you hardscape together with it using smaller Pc of rock to fill voids. The stuff can be molded if you work quickly. Dry Cap sand could be worked into the exposed surface.
You might want to watch this: http://www.youtube.com/watch?v=wZDexLMZFrU
James Findley has several tanks on you tube that employ some slope/elevations, maybe you'll get an idea there.