Re: molding rocks,
I've tried most every way to mold caves and free forms and still find the quickest and most dependable is molding on damp sand, I make caves just rounding up the internal size I want, and mold free forms over damp sand spread on a sheet of plywood, doing this you dont get that flat edge of the caves or the flat plates,
you can make a rough surface to the sand, sometimes I will lay ziti or inverted pcs of shell pasta in the pcs, you get the shell impression molded, thick pcs often get straws through into the sand, they pull easily out of the dry concrete, and good sized crystals of salt will make a hole looking like a barnacle,
very easy to make very natural looking rock thats hard to tell from real corals. I have tried at times dripping various dyes over parts of the rocks but find its more work then necessary and cultured coraline algae looks much better.
I have never had any problems with using oyster shell, for me its cured no differently then crushed coral. Maybe its the proportions in the mix. one part cement, one to two parts oyster shelf, one to two parts sand, and occasionaly some rock salt crystals put in place.