Can I take legal action against my family for upsetting my mother with Alzheimer's?
You ask about your Mom with Alzheimer's and the abusive phone calls she gets from relatives. From your question, I'd like to focus on this phrase: "Maybe I'm supposed to interfere since I'm her caregiver and supposed to be protecting her." "Maybe I'm supposed to interfere since I'm her caregiver and supposed to be protecting her."
The advice is yes, you are supposed to interfere and you do need to protect her. Family members with mental health issues are going to be concerned with themselves only. Perhaps they can't help it. Whatever their motivations are, it doesn't matter. You can't reason with them, as you've tried and found that it didn't work. Now it's time to change the rules and keep them from contacting her.
First, you need to remove the phone from the locations where your Mom has access to it. It may be inconvenient, but you need to stop her unlimited, unsupervised access to phone calls, as it is clear that these upset her and cause unnecessary distress in her life. If you need to change the phone number, do it. If you need to use a cell phone only, do it. Things are not the same as before and you need to make your home environment accommodate mom's needs, which have changed and will continue to change. You can protect her best by cutting off the ease by which inconsiderate or abusive family members have contact with her.
Next, I would write a letter to all those family members who have been upsetting your mom on the phone and let them know that starting now, you will be monitoring all phone calls she gets from them. All you need to say is that she is unable to handle their ordinary conversations and that you need to limit her calls because she gets so upset after some calls. Don't blame anyone specific. Just set firm limits. And stick to them.
If you're busy working during the day, let them know that they can only call when you can listen in and when you are available to keep the conversation appropriate. Give them days of the week and times that are specific (e.g., Sundays at 6pm, Thursdays at 7, etc.) and that work with your schedule.
You don't need to record the conversations, get others involved or take any additional steps unless this strategy of cutting off easy access and setting up a schedule so that you can monitor calls does not work. If your family members don't respect your limits and your word, then it's time to explore the option of getting a court order to restrain the verbal abusers. See an elder law attorney if you reach that point, and get advice about the best way to handle stopping further abuse.
Thank you. That's pretty much the responses I got. Someone even said calling relatives had update a parent so much, the parent died. It's horrific what aging does to a person and horrific what families do to each other. Sad. Thanks again so much for the input. Sometimes, you start to question your own reasoning when you are sunk so deep in the forrest that you really can't see the trees.

