She doesn't seem to have any problem in backing out of agreements she has made with you or her son. She just expects you to pick up the slack she has caused.
So she is spending more and now says you will need to pay her part of the tuition? Nice. More than once she has said that her expenses will be more than yours. How does she know this? (You don't have to answer.....I'm just making a point.) It shouldn't matter, if she made an agreement. Maybe everything needs to be based on the sliding scale.
I don't think you are going to be able to simply take her verbal agreement on these financial issues. She doesn't seem to stick to much of anything she's agreed upon.....whether it is scheduling, finances, or what she tells her son. You may have to get the court to decide about the scheduling and the finances. (Somehow, I think she would still try to manipulate her way around things.)
So sad it is this way. If she was not this way before she became a WAW, then hopefully, she will stop this wishy-washy style some day.
Do what you believe is the right/fair thing to do. In doing so, you won't be out to punish her, and you won't be putting yourself and S12 at risk financially.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!