Kathy

My sympathy for the terrible predicament you face. It is just not fair. My opinion is that if your Mother is mentally competent, it is her decision to make. While there is no disagreement with David's post, sometimes the quality of life after TX is such that without a major compelling reason to deal the "new normal" , it is reasonable to seek hospice care instead of the aggressive treatment (TX) base of tongue cancer requires
I would ask the doctors what they expect your Mom to be able to eat or swallow after TX
Will she be able to speak so that others understand her? Will she be on a permanent feeding tube? Will she be frailer than ever? Who will be her caregiver during TX?
Realistically, do they expecta "cure" or just buying time before a recurrence? Don't be surprised when they say "we do not know" but ask anyway.
If my cancer comes back in 15 years, I might well choose hospice (if there have been no breakthroughs in TX ) but since was only 60 with a wife and son dependent on me the first time and 61 the second time, my decision was easy. Your Mom's decision does not seem easy at all but very difficult and based on what has happened to me, I understand her reluctance.
Just a different view to consider in case your mom declines aggressive TX Your mom probably fears losing her independence permanently and not being able to garden or take care of herself as a result of TX. Finally since even her doctors share your fears that the TX will kill her in her weakened state, do not be pushed or bullied into thinking that you are a bad child if you do not insist on TX. Again while you are a caring daughter , your mother is an adult and at this time needs your support of whatever decision she makes
Of course you want her cured but at what cost? Hospice can deal with the pain for the patient but not for the caregiver. No easy answer here
Charm