I often have to cut out a nail on box either to find or fish another wire. Just pry it away from the 2x4 (use a 6" steel scale to find out which side the stud is on), and cut the nails with a metal cutting blade in the sawzall.
Pull in the new wire, and use an old work box - no sheetrock damage if you're careful.
In my neck of the woods, sheetrockers never return, no matter whose fault it is.
Re-reading your post, I'd use my toner. Kill the main to eliminate noise - can usually find it in the room and pinpoint where it is. Sometimes a feed is missing, or not pulled into a box - toner sorts it out real quick. Part of the trick is to kill the main - often too much noise (hum) - have had to resort to headphones on occasion.
Grov
