If the spaces on the "mittens" are not present in the original DEM file (which they may not be, due to DEM resolution) then they will be extremely difficult to reproduce in TG2. To see whether they are there in the original data you want to load the .ter up in a new, blank project, with no other displacement applied, then turn off the "fractal detail" (Fractal Detail tab in the Heightfield Shader). If you see the gaps then they are present in the DEM data and if you keep the additional displacement to a minimum they should be maintained. If they're not there in the original data, I would sooner try editing the heightfield in another program such as Wilbur or Leveller than try to create such a specific shape in TG2.
With the populations, I would again start by going "back to basics". Try to apply a single population to the basic terrain, without surfacing or additional displacements. If this works then go from there, one change at a time, toward your goal. As soon as you make a change that messes things up, you know why it's not working and you can either experiment with it further to address it, or report the problem and see if others can help. More specifically, in your .tgd I see that several of the populations have a density shader with an extremely low coverage. This will produce a very low probability of seeing any objects. I would suggest not using the same density shader as your fake stones since the density functions work a bit differently for populations. Again, start with full coverage, then apply your density shader and use high values above the intended range first, to make sure you can still see your objects, then slowly dial it down until you either get the coverage you want, or you lose the objects entirely. If the objects disappear, back off on the settings a bit and try again. There is often a lot of fine-tuning necessary when working with very specific needs for your scene, especially with a small range of altitude as you mentioned was present in your original terrain. There's really nothing that will let you avoid the need for experimentation to get the exact result you want.
- Oshyan