Brilliant idea for a diorama! Really curious to see how this turns out.
The ideas sounds good and close to the book so far.
Using very fine sand sounds like a good idea, but I would personally have every piece of ground covered in grass (different colours and lengths, of course though). To me, that sounds more realistic next to the Trees. Only exception might be a white sand path, or a road paved with large stones.
The colours you describe sound good. I think it will be largely unimportant if it's all covered anyway.
Alternatively, it may be a nice idea to use teddy bear fur:
https://elladan-shop.de/epages/b0930d3e ... elaende%5DAs for flock, ideas also sound good again. Maybe, instead of using gold or silver flock, you could also drybrush the grass a little after gluing it down. I would recommend to experiment a little on smaller pieces to achieve the effect you like.
Making the trees will be the most challenging, I think. You could try something along these lines:
https://hdrskirmisher.wordpress.com/201 ... ondorians/(but with the tree tops on there, of course).
You could use Woodland Scenics tree armatures as branches, and then glue leaves on there. For leaves, I can strongly recommend these punches:
http://www.greenstuffworld.com/de/suche ... it_search=Grab a green piece of paper, quickly paint it with watered down greenish paints, let it dry, punch out leaves and repeat. Creates beautiful and realistic leaves!
Good luck and keep us up to date!