Did it ever occur to you that you will be doing more for your H and even your M by saying no than by saying yes?
I agree that protecting yourself is the most important thing, so let's start there. H will say you are punishing him; that's the MLCer outlook on everything. He also said all his problems are your fault, I assume. You know you are not punishing him. You know you need space and you know you sorely need treasured time with friends. So ignore his interpretation and know you are protecting your solitude. So if you need that solitude to heal, as you did hiking with your mom, know that that healing is the KEY to your standing. If you are exhausted or anxious all the time, you will burn out from standing or you will not be able to do it with a clear head.
Now for H -- Your choices can't be there to teach him anything except that you have a boundary, and he might not learn that anyway. But you still have to do it, it may be the only thing that will work on him. He is a teenager. If your teenager drove drunk, would you give him access to the family car the next week so that he would feel you trust him? Your H wants to drive drunk (MLC metaphor) into your party. No thank you! Not good for H, for you or for the party guests. He can design a very special birthday activity for his kid without you, he is choosing that.
Now if you kid really really wanted H there and you had the strength to do it, I might say go for it. But even then I might not. My D wanted to bring my dog to her dad's house and I said as gently as I could that the dog lives here and that her dad can always get a special pet if he wants one. It's awful but I think it's ultimately good for my D to also see boundaries. The whole situation stinks but clarity on boundaries is a good way for our kids to to not wonder and worry about so many unknowns. Things are known and then they too can work on acceptance.
I stood for seven years and my H lived there for five of them. I assure you that no amount of saintly resolve will change them. Some of them do change, and I believe in restoration with all my heart and mind. But I thinkboundaries might be the only chance for the LBS healing and thus for the potential restoration when/if the MLCer ever takes his mind back from the Alien Forces who took over his body.
I believe I will see the bounty of the Lord in the land of the living. Wait for the Lord with courage. Be stouthearted, and wait for the Lord.