Anyone have advice on how to deal with my mom's anger?

Yes, get her checked for a UTI. Also, can you take a video of your dad next time you're visiting him, have him say "hello" or speak to her or whatever he can (if he can). Then you can show it to your mom whenever she wants to "see" him.

We thought my mom was sundowning when in fact she had a serious UTI, treated with IV antibiotics. Her neurologist suggested melatonin 3 mg which helped and now only give it to her as needed. My mother is sweet as well, but when we wake her in the middle of the night for changing and turning she yells and screams so loud it wakes everyone up. It is upsetting, but it passes and she goes back to sleep. Calm manner, gentle massage, dim lights, and soft singing usually helps. My mother is the quiet type, never raised her voice for the most part, this is difficult to witness, but it seems it is part of the disease process. Wishing you the best as you continue on the caregiving journey with your dear mom.