1 - I’m not familiar with the Datasmith workflow, you’ll get more help in the Unreal Studio Beta subforum.
2 - Realtime engines can deal with millions of polygons easily, but super dense meshes like trees, grass, and even small details (like a beaded pillow or crystal chandler), are likely not optimal and could cause more problems than they are worth.
3 - There’s a foliage painting tool, but you’ll want to build and manage an asset library. You may want to make a default one that has generic assets that you use for every project.
4 - Game engines have almost entirely switched over to PBR. A standard material will have a base_color (diffuse), roughness (invert of glossiness), metallic (where the surface is metal or non metal), a normal map (instead of a bump map), and an ambient occlusion texture. You’ll want to closely follow the PBR guidelines for base_color, there’s charts and references online. Instead of pure white for white paint or snow, you’ll want to use something closer to .8 and something black like charcoal at 0.02. Metals completely rely on the base color to look realistic, you need to use the suggest values to get anything realistic. Leave the specular input alone for most materials, instead turn up the roughness. Metalness should be generally treated as a black and white mask, or use constant 0 or 1 values. Painted metal is not metallic, completely rusted metal isn’t metallic, but you can use a bit of gray to transition from metal to non metal where there’s rust fading in and out.
6 -An actor could be a light, a blueprint, a mesh, or anything that can be placed into a level. A constant is a number, there’s also constant 3 vectors, generally I’ve heard of constants being used in the material editor. Constants are typically something you wont be changing or adjusting. A parameter is what you want to use in place of a constant if it will be tweaked.