You can't help mentally ill people. They can't face reality. It sounds like your mother has alot of issues. She is blaming you, because you are the scapegoat. She is an angry person inside, and is throwing that anger at you two becuase you are the closest ones around.
You two need to get away from her. She is toxic. The Jehovah's Witnesses are a symptom of her toxicity. Remember her for any (if any) good times you had. You two did your best, and have to remember that her behavior isn't your fault. The congregation ain't going to do crap for her.
The next time she gets into issues, call elder care and have her committed to a psychiatric institution, even if its' for a few day watch. Keep & document anything she says that's not normal, and anything that is remotely threatening to herself or others. Your best bet is to get her into a nursing home.