I'm glad to hear you have a safe and comfortable place for yourself and your children while you work on the relationship with your husband.
I'm also glad to hear he's in DBT. It is a long program, but it does work.
How do you validate feelings without someone thinking you are agreeing? Or was he just hearing what he wanted to hear?
In the end, he will hear what he wants to hear. I'm assuming that you understand that validation is acknowledging his feelings, not agreeing with him.
Validation is a subtle tool, and best used when you are feeling calm and caring (otherwise it will be insincere, and not work), and when he's calm enough to be receptive. (I suggest you practice it on people in your life who don't have BPD as well; it is easier with them.)
Using validation to end a rage or dysregulation is very difficult so don't be surprised if you fail. And if it doesn't work, be ready to remove you from the situation and let him work through the mood himself.
One of his worst rages was when I tried to implement the plan of taking a break when things got heated and I left the house. He caught me in the driveway and came unglued!
That's not unusual. Consider what is happening: His internal, emotional world is in turmoil, with intense feelings he doesn't have the tools to deal with well. His favorite coping mechanism for that is to blame it on you, and use you as an emotional punching bag to deal with his feelings.
When you go away, you are taking that away from him, and he's stuck without a way of addressing his feelings.
He doesn't like it, and will do all kinds of things to stop you.
If you are consistent in getting yourself away, it will be good for you (Being an emotional punching bag damages you!), good for him (He has the opportunity to find other ways of coping with his feelings, hopefully better ones, especially since he should be learning some in DBT), and good for your r/s (This kind of conflict just damages your r/s without resolving anything).
Do you need more help getting yourself out of these conflicts/rages/dysregulations? If so, describe what leads up to a typical one starting, and we can help you with solutions.