Please, please, PLEASE take your kids and get out RIGHT NOW.
Forget gathering evidence or planning for a long-term future. Your husband sounds DANGEROUS. Go to a shelter if you don't want to go to your mother's.
Do you have a friend who could come over to help you pack while your back is injured? Do you have a friend you could stay with in an emergency? Because this sounds like an EMERGENCY.
The part about him injuring you--deliberately???--during the back rub compelled me to write this. This situation is WRONG and you must GET OUT IMMEDIATELY.
Go to your mom's for the short term, and don't sign anything or let her bully you. (Yes, she does sound insane as well, but she will not physically harm you--and your husband already has).
Don't worry about disrupting the kids' school routine. Getting all of you out of this situation is WAY more important than school. Missed school can be made up later. Kids can recover from moving to a new school or whatever. They cannot recover from something happening to their mother because their father is INSANE.
Please, please treat this like the emergency it is and take yourself and your children out of this house. Even if you must go to a shelter--which may help because they will have resources like lawyers.
Don't distract yourself by regretting the last 10 years. You've gotten two children and a lot of life wisdom out of this.
Now is the time to go somewhere safe. Then you can begin divorce proceedings and get advice about how to obtain full custody of your kids.
I liked your comments about your views on polyamory and relationships. I know that you will someday obtain the life you want. But that's an issue for later. Now, the most important thing you need to do is get away from this sadistic, controlling man.
You sound like a strong, smart woman who will be able to find the inner resources to get out of this.
Please hurry. This situation is NOT OKAY.