My Grandmother is a similar age with complex medical issues and requires a high level of care. Like you our family is dispersed and its hard when you're not there on the ground seeing whats happening. You can only do so much over the phone. A lot of the time with my Grandma she sounds quite good on the phone, mainly because she's buoyed by the phone call. So its hard to get a good indication what she's like during the day. You need someone on the ground there you can talk to you can trust to tell you whats going on. What are her care arrangements now? Is she living alone or does she have some level of assistance?

As for talking to her about it, in my experience the biggest need the elderly have is the need to feel safe and secure. While we might stress about different aspects of treatment, Grandma's biggest worry about hospital was she was not in her own bed surrounded by her own things and own routine. She finds it all very unsettling. Sometimes with bad medical news she goes into denial, and we have to work through the issues with her.

I think you're on the right track. Being patient, asking the right questions and getting informed. Keep the dialogue open with the medics, your mother and the rest of the family and work through the issues as they come up.

I do feel that you should be talking to your Mother about this diagnosis.
She is obviously aware of the diagnosis and I am sure that she would want to discuss this with her family. She will need to know that you will be there for her , advocating and supporting her. You do need to talk to her to find out what she wants. This is the most important thing. What, She, wants.
Once a treatment plan is offered, you will need to talk about this with her. What is involved. How she might experience this. What side effects there may be. What is the likely outcome and what the longer term consequences may be. What her quality of life will be. You really need to discuss these things and what she really wants to do, given her age. Make sure that she really does understand all this. Does she want treatment or is she going along with what her family want or expect. To me, it is paramount that you do understand exactly what your Mother wants.
Not easy.
Tammy