Yes, it is still a pipe thread. The pipe dope is there to fill in the gaps in the thread. It is impossible to machine the treads close enough in pipe to not need something in the threads to make a seal.
Yes and NO depending on what kind of piping system. for drain connections yes like tub drains or fixture traps but you need access to all slip/compression fittings. and NO you can not use PVC to...
First, clean the inside of the fitting to be soldered, and the outside of the tube it is to be soldered to, with abrasive cloth, steel brushes, steel wool or equivilent.
Second, apply a thin coating...