This man's thinking is more warped than was apparent from your first post.
He volunteers you for things you don't want to do and without discussing with you in advance. In other words he's quite happy for you to be unhappy, he even organises things so this will be the case.
You are afraid of even discussing it because it will spark a "blazing row".
Your mother hugely over reacts to comments which she feels call her perfect vision of herself into question (very familiar territory in my own life!) and punishes you excessively with the intention of stopping you doing this ever again.
You have now been trained that its your job to keep other people happy, and feel massive guilt when you don't.
Even using grey rock with your mother has been a huge challenge for you as she has escalated her emotional blackmail. (This is what these people do if they feel their "victim" is slipping out of their grasp, it's very difficult and you really need to stick with the grey rock).
You feel you need to find the assertiveness course that's exactly right for you - but maybe you should just start with "an" assertiveness course, see how you get on, and then perhaps see if you could find one that meets more specific needs?
Your husband trying to drag you into punishing one of your children due to his displeasure with said child is really chilling and you did well to stay out of it. Otherwise your children would be learning that if they don't keep dad happy, not only will he be horrible to them but you will too. I think this is really disturbing.
Interesting that you had used a phrase like "enforce a consequence" and he "apologised" this morning. I wonder if there's a little voice in his head telling him you're copping on to his behaviour and he needed to reel you back in? In any event, the voice didn't hang around long enough for him to control himself when you didn't accept his apology with gushing thanks.
You can see that he's trying to bait you which is very useful. I get the same with my mother who often trots out "I don't want to talk about it any more, you've upset me terribly" sort of shite. Because she lives a couple of hours away from me I can tell her "ok, I'll call you in a week and we'll discuss it then". Then I don't contact her in the meantime and I call her asking how she feels about what happened. She has now learned that I won't be put off so she may as well engage in a conversation. So I listen to her (usually completely warped) version of what happened, then tell her my side, using words like "when you say x, I feel y". It has helped a little but I still need to keep contact low.
When you're living with someone it's a lot harder. This man's treatment of you is unacceptable, and while it may never change, it certainly won't until you start tackling it.
For me, what has helped is setting boundaries in advance, you could try the same - e.g. "if you volunteer me for anything else again, I'm going to book a spa treatment for the day instead", "if you try to make me punish my child just because they have annoyed you, I will take them out to McDonalds for a treat". Then when whatever happens, you follow through. Do not feel that these are threats - you have the right not to be forced into doing something just because someone else has decided you should.
It seems like you're being caught on the hop a lot, my mother has done this to me too, so I sympathise. I'm still learning how to preempt her plots and not always succeeding; however when she puts me in a situation I don't want to be in, I just walk away so she hasn't really achieved what she wanted.
What I try to do with everything she says is ask myself "why is she saying/asking that, what's behind it", because it's rarely a straight and honest comment/question, there's usually a plan. I try to give myself time if possible, "I'll call you back", and consider what she might be at before giving her a response. Very frustrating way to live!
Sorry this is so long, I hope it's somewhat helpful.