1 & 2. For scanning small objects, the SR300 camera model is best suited. The R200 camera is better at scanning larger objects and the human body. The minimum scan distance of the SR300 is 0.20 m, whilst the R200's minimum distance is 0.30 m.
I have seen what appears to be closer-range scans such as skin, though there may be increased noise, lines and surface break-up on the scan. Some people get around scanning quality problems by generating what is known as a Point Cloud and then converting that cloud into a model.
With the SR300, you can use an instruction called SetIVCAMFilterOption that enables you to alter the camera's depth scanning parameters to provide high accuracy at close / very close range scan.
3. You can get the color information by converting your exported .OBJ model into .PLY format. The .PLY file can then be imported into a modeling package such as Blender for further editing. The article below explains how.
I have no information about how a model's colors will be affected when imported into a particular package. For example, I found in my own work with the Unity game engine that far more of the original color and texture information was retained if I imported the .OBJ model directly into Unity instead of into a package such as SketchUp and then exporting it from SketchUp again before bringing it into Unity.
5. I was not able to find demonstration videos of RealSense being used to scan jewelry, unfortunately.