She immediately redirects the conversation back onto my dad and keeps blaming him for her really really depressed state.
You can't expect your mom to comply with your boundaries. You have to enforce them. When she starts blaming your dad, you need to say something like:
"I love both you and dad. I refuse to be put in the middle of what's going on between both of you. I won't discuss this any further. You need to take this to a couple's therapist and discuss things from both of your perspectives. I'm not a therapist. I'm going to get off the phone now (or go home now). We can talk/visit on another day, but not about you and dad."Expect her to keep trying to have the same conversation. She will likely get angry with you. You will need to enforce your boundary over and over again, before she even thinks about complying.
Someone has to be honest with her and about her problem behaviors. Appeasing her for so many years, and sweeping her problem behaviors under the rug, has resulted in the behaviors you now see. Your dad needs to be cautious. Your mom's paranoia could lead to your mom making false accusations to the police, about your dad.
Some people can be very good at fooling therapists. Consider the following:
1. Contact her therapist and advise the therapist of the situation, your mom's history and her current paranoia. You could ask the therapist to recommend a couple's therapist. Hopefully, the therapist will skillfully interact with your mom (based on what you tell her) and will refer your mom to a couple's counselor (in addition to her individual therapist). A skillful therapist should be able to handle this, without your mom knowing you called them.
2. Suggest that your mom ask her therapist for a referral to a couple's counselor or family counseling. If your mom & dad can proceed with a new counselor together, that would be good. Perhaps, it would be beneficial for you to briefly join in. Thinking what needs to happen is to have a family/couple's counselor who will start out with speaking to each person individually (to gain everyone's personal perspective), then proceed with joint sessions.
Your mom needs a dose of truth. You say your dad has finally set boundaries and is enforcing them. Is your mom just NOT listening to what he has to say or is your dad NOT laying out the issues and reasons for his boundaries effectively? Have you ever told your mom that it takes two to have relationship problems and that she plays some part in that?
If you mom refuses to accept what you dad is telling her, maybe if he puts it in writing, it might sink in for your mom. (as long as it isn't an abusive rant, but something factual & honest)
Something in writing, could be something for your mom to take to her individual therapy. If she can share it with her current therapist, there is a chance that her therapist will be able to see the main problem is your mom. Your mom's objective in sharing, would be to show how crazy your dad is. Hopefully something in writing from you dad could provide enlightenment for the therapist.
Your mom needs a major dose of truth. If you dad isn't able to manage the situation effectively, you may need to step in (unless there is some other family member suited for this). How do you feel about contacting her therapist?