No doubt. If you dig a little deeper with a lot of therapists they have a history of trauma, as you will know.
Back to the original question you posed, which was why does he bite his nails. Reading through what you've written this is the most benign of his disturbing behaviours, but a reason would be his serious MH issues and treatment resistent depression, surely?
No doubt your DC find it disgusting, they are also presumably disgusted by his damaging rage tantrums, entitlement, paranoia, anger foul and abusive language towards their mother.
You walk on eggshells, you are terrified and exhausted. What would your advice to a client be if they came to you with this picture?
He has lost every relationship he ever had (except me and DC) in the last 7 years, and rightly so. I'm wondering what makes you think that you and your DC aren't worth more than this? What will it take for you to get you and them away from him into a less damaging space?
Do you have a reason that you can't seperate from him?