The races and genders are separate purchases that can be added to the rig, so that they all use the one mesh. There’s a brief overview of each on this page.
You can change the topology, but it might require some weight painting and editing of the corrective shape keys in highly deforming areas (e.g. elbows and knees). Changing the face topology is generally best avoided, since the shape keys there are very sensitive to changes.
Multires doesn’t change the topology though, so I recommend either using that, or baking displacement maps from a separate mesh. Some weight and shape key adjustments might still be necessary though if the sculpt strays far enough from the base shape.
The rig was made without regard to retargetting, so any compatibility with those tools would be coincidental. I might look into what’s involved in supporting mocap data at some point, but for now you’d need to take those matters into your own hands.
Not at the moment - I don’t know yet whether they’ll be included as part of the textures, which will most likely be sold separately.
All the functionality is covered in the reference charts, but yes some workflow examples would be helpful.