Is holding saliva in her mouth end of life?

Is your mom on hospice? If so, please speak with her nurse about her condition. Hospice nurses are excellent caregivers who specialize in end of life care.

It is very common. It is difficult to swallow toward end of life, esp when any dementia is involved. This can become problematic if any food is forced, as food can be swallowed into the lungs causing aspiration pneumonia. You might ask your doctor if there is any good reason to have the swallow evaluated by an OT consult. This can often be done in facility.

Maybe. Many people lose the ability to swallow properly (dysphagia) and as time and disease progresses that can include their own saliva - this will most likely eventually lead to aspiration pneumonia, a condition that often ends life. My own mother struggled with this for several months before succumbing.
(It's worth noting that aspiration pneumonia can sometimes respond to treatment if you choose to pursue it)