Well, I can't answer the first ones, and I haven't preheated the quenching oil with the old carbon steel I've retempered. Maybe it is an added safeguard against cooling down so quickly as to cause the steel to shatter or crack. While I did have a chisel crack when quenching in water one time, I've not had that happen with oil quenching.

As to tempering after quenching, shouldn't be a time limit involved. As to cleaning, that is a neccesity along with using an abrasive to clean back to shiny metal if you are gonna use the color of the heated metal to judge your tempering.