Hey Tkane,
  I do think the scale of the trees is not quite right, they should be smaller I think and there color should vary more. Maybe have 3 or 4 different trees in the replicator. Other than this your image is pretty nice , I like the lighting in your first attempt the best though.
  For your other issue about not getting the 10,000 trees , there is an issue when using a shader with black and white to control where the trees go . It is like if you want the trees to cover 50% of the terrain then you roughly can only place 50% x 10,000trees , so you can only get 5000 trees. This gets worse when you want a small area only to get trees .
 What you can do is Create and Edit  a new master of your terrain , then convert your new terrain to a vertex mesh and use the Paint brush select  tool in the Vm room to select where you want the trees to go.  Give this selection of polygons a Shading domain name .
  Then in the replicator window , use the new terrain as the source object and use the Limit to Shading domain and select your Shading domain you just created in the vm room.  Now your trees should be densly populated and get much closer to the 10,000 trees.

Hope this helps.

Mike