Regarding the residue on the piston skirts, many times this accumulates during the initial break in process. Push an engine too hard too early and add a bit of extra heat and you will get this. It really doesn't hurt anything as long as it doesn't get any worse. My bet is if you clean the piston skirts before reassembly, even reusing the original rings (if they sealed well) this won't recur. Skirt staining is a good indicator of the engine's history though.