Quite honestly, that is why I stay. Because as much as this sucks for me, they are still happy.
Happy does not always equate with healthy. Momentary "happiness" to your kids may not be teaching them what parents are Responsible for teaching them. And also keeping them in the necessary Child role. Your son seems to have way too much power in your household. For instance, you mentioned your S5 didn't want to go see you at karate. Therefore, H did not take him. That Shalom in the Home guy was just on Oprah talking about something similar. One of the kids wanted to go to a concert but his Mom was graduating college and he had a fit because he didn't want to go. The rabbi told the parents that they have the responsibility to teach the children empathy, respect, etc for the parents and that kids always getting their way was going to make them spoiled brats. The parents Told the son he was going to the graduation because he needs to show some selflessness towards his mother, etc. Why are you letting a 5 year old decide where to go? You make the rules not him. You need to regain some control as a parent. You talk of H lacking empathy for you, don't your think your S is picking up on that too. I also feel like your H is using your S to his own advantage, pitting him against you it seems. That is so destructive. And don't you have a D too? Where does she fit into all of this? You always mention how close S and H are, but I never hear about him and D. Anyways, as the kids get older, they are going to be even more confused if you and H do not come together as a parenting "team", whether you are M or not, you need to have firmer guidelines in place. You seem to be carrying a lot of guilt about the A. Don't let that ruin the R you have with your children, even if the one with your H is so dysfunctional.
