Fml,
I know you feel your life is out of control, but look at the big pix, it really isnt. Years ago you "saw" the writing on the wall... as ah was going down. You made a plan. You worked hard at your job,(check) worked at finishing school (almost check), knowing what your future was going to be for you and your child, you had a plan. You have continued to follow your plan.
Now that you filed, stbxah has checked into rehab. This doesn't devert from your plan. It has really nothing to do with your plan. Rehab wants you to adjust your "plan" to help stbaxh. Addiction and sobriety is a one man show. It is early in his recovery and I dont think you need to be present either. You filed for divorce, you are done.
Now to be treated like shxt by the rehab staff, I am just not feeling it. You owe them/ him nothing. Let him figure out his issues, he's alone now, by his own actions. I am sure you have threatened divorce more then once, or twice or maybe 100 times, this is not out of the blue. I hate to say, but it is probably best for him to know that you are not there for his recovery. It's to late. As we all know, chances of his first rehab "working " are slim.
Continue with your plan. Don't keep adding a new "day ones" and continue engaging with him or them. He is a grown man and can figure out how to fix his problem without you enabling him. You are doing fine. You have a plan, sometimes we have forks in the road and we need to take the right road, continue forward as more will be revealed, its not over yet. Hugs!!