OK - You take your power back by removing yourself from the situation. You start to move forward by making you and your life better. That is the best way to take our your revenge. Cutting my D's hair doesn't even cross my mind. It is something that my XW has always handled. Yes, she told me to take it off the child support because it was $100 and if she didn't offer that I would have asked her or told her I couldn't afford it. Your sons haircut costs maybe $20 it was not about the money with you it was about control. I see you trying to hang onto anything you can. You have to let go of the edge of cliff and just fall.
I told you early on that you may have to give a little to get a little in return. Your W has to realize that you are a safe place and if she is comfortable with you then you can get to the point of a good co-parenting relationship. In order to do that though you are going to have to take the high road on some things.
You are going to be D'd very soon.....IMO there are little boundaries that you will be able to enforce so if being a good co-parent is your goal then know that respect goes both ways. You may have to give it first in order to get it back.