I finally said, "S, just say goodnight and don't worry about it. We will get up early tomorrow and I will drive you to the other house for your shoes, then bring you to school, then bring D10 to school, then get to work. D10 will just have to get ready early with us and come along. You need the shoes."
FL, you know how she is at this point. What you should have done was listen to your son in the first place. He asked you - he told you that it needed done and in these circumstances we have to do these things. Lessons learned. We have to learn that our X's are out on their own planet. If there is anything you can do to make your son's life easier, then you have to do it. That way you take her out of the drama. And you make your son's life easier.
At this point, you have to know that any disgression of yours is a point she can make her own. If you take that away, your son's life is better. That's the key. Just make your son's life better. Sons remember these garbage days. (I have 3 sons FL - I know) Give yours as little as possible. Make him remember how you made things better.