Hi!
After i saw how laborious the process of instancing is i can make a suggestion... can you perhaps write a script (like an additional function) that generates randomly spread out (onto a plane) objects such as stones and plants. I think once you managed to write scripts for generators such as marble and turbulence, this should be a piece of cake for you.
I believe this idea would be a huge step regarding the instancing procedures (that are painfully repetitive therefore pain in the ass for creativity).
Cheers!