A. Not really, you might have scratches that need to go across seams flawlessly and doing that in photoshop is a pain in the ass. Plus its easier to visualize on the 3D model where the wear and scratches will need to be. B. It's really not that hard. Planar maps or auto unwraps based on smoothing or angle will cover about…
Yes, for normals, AO, and additionally a color mask. I apply basic diffuse colors like RGB, or RGBCMYW to materials on specific polies, this lets me easily extra layer masks in photoshop for different material types(does not require uvs in most apps).
Personally, I do not like to do this, for a few reasons: A. Its super easy and fast to do this in photoshop, or 3d paint a mask in mudbox etc. B. Your highres needs to have some form of uvs C. You dont get an easy to use mask for the diffuse and specular areas, as you would with standard techniques D. Much less flexible,…
Generally, you want to model things in the low poly that affect the silhouette of the model (within a certain amount, say, an inch or two). That amount depends greatly on the prop - the bigger the scale of it, the larger that amount would probably be; the more important a certain detail is, the less that amount would…