Wow, I would not be pinching the child back. I just don't think that's ok at all.
Surely the point is to teach the child that pinching is wrong, inappropriate, that it hurts. How will a child learn pinching is wrong when his/her mother is pinching them? #
You say you have tried all of these different methods but for how long did you try each one? The time our for example? Consistancy is really important, so if you tried one method a couple of times, felt it didn't work so moved onto another then the child will get confused.
Keep going with one consistant method. Personally I would get down to the child's level, hold he hands in mine and firmly say "No, you do not pinch mummy". If she continued I would then sit her in a time out and again, at her level, explain, "No, you do not pinch mummy. You need to sit here for 3 minutes because you pinched mummy again." If she moves from the time out before 3 minutes is complete, take her back and start the time again. After the time has ended, ask her to apologise.
Follow this same routine EVERY time she pinches and she will soon learn that there are consequences to her behaviour.