SadMummy I've read this thread with interest as I too battle with a challenging ds of 5. I liked Leo's general approach to parenting but really disagreed with what she said about you "locking horns". As I read, I was thinking how reasoned and open your responses were - I just wanted you to know that.
I also want to say that I think you are a great mum. I'm not saying it lightly or to cheer you up:I'm also a SAHM and the list of things you do with your ds had me gobsmacked. It's so, so hard to be present and loving and interested and engaged all the time - well, it's hard for me. Well done for the way you are parenting, and your thoughtful approach to it all. Your dc's are lucky to have you.
Things have improved between me and my DS recently and I'm offering what worked for me in the hope that it will help you. Please just ignore the following if it doesn't feel right for your situation. Also, this should come with a major psycho-babble warning. Again, please ignore if it's not your thing.
So here goes:
Given all that you have tried to implement I think it's safe to say that it's not working, and that your DS is not going to change: YOU have to change. When I say you are the grownup I don't mean to sound at all critical or patronising. I mean that it sounds as though you are in the place I was in: basically scared of the little boy you love, and try so hard to parent well. Scared because you have no faith is his, or your, ability to control his behaviour when you are out together, or in company. It's awful -I've been there often and to an extent, am still there.
But if I am right, then your child is more frightened than you are. It's terrifying for him, too, to feel that Mummy is scared and so he pushes harder and harder and acts more and more wildly to find the boundary, the point where Mummy will contain him and he can feel safe.
So that's what I mean when I say you are the grown up. You have to find a way to decide deep, deep inside your self that you WILL NOT allow this behaviour. Forget the charts and rewards and punishments: all that counts is that you really do believe yourself, and believe in your own authority. Until you believe yourself, he won't believe you. I could only get to this point with a lot of support, mainy from my DP and my best friend. If you can't believe in your own authority (as I couldn't/can't) then keep seeking help until you can: your DH, your family, your friends, therapy, counselling, books, parenting courses....just decide you will do what it takes, and won't stop till you have found a way to believe that you CAN HELP your ds. Keep reminding himself that his six, that he is a little boy and that you are still his universe: you can fix this for him and for you.
I would have sought counselling or therapy if I could, but I am in a third world country with very limited resources. But I needed, and need, all the support I can get.
Please believe me that I empathise strongly with you, and that I have spent/do spend a lot of time feeling awful about exactly the issues that are bothering you. I'll watch this with interest to see if there are any tips I can pick up. Good luck and hang in there: there are a lot of us in the same position.