Sorry you're in a tough situation today. I can imagine the pain when the van shows up without notice. If you're in your bedroom right now, take a few minutes to slowly breathe in and out with your eyes closed. Take control of yourself again.
The fact that his Mom showed up and is deciding who he can and can't talk to tells me your spouse has a lot of work to do. His Mom is controlling the situation, which is not healthy for him. Do you really want to live with that? "Do you want him to start drinking again? Do you want him to relapse?" STUPID questions. Of course you don't!!! But that's not up to you, and you aren't to blame if he does! You didn't Cause it, can't Cure it, and can't Control it.
Doesn't sound (to me) like he's got it together yet. And Mom isn't helping. My personal feeling is you need the distance as much as he does, even if you don't see that yet. You need peace and calm to start on your own path of recovery.
There needs to be communication about how to handle the finances, child care, etc since this is a formal separation. To say the kids can come over whenever they want won't be helpful to you in the long run. You need a plan. I would suggest talking to an attorney about how to handle a seperation. You need to start taking control of some of this, and get things in order for YOU. Seems like he's in the drivers seat (or passenger seat and Mom is driving) and you're not making any decisions for you.
Talk to a lawyer, go to a meeting. You're going to be okay. You're going to be okay. You're going to be okay.