Dear Worried Daughter,
Welcome to our community! I am sorry to hear your mother is having to battle such a rough cancer after two successful battles and especially at her age. She is so fortunate to have you by her side. Just keep in mind, the best baseball players only hit the ball about 30% of the time. I admire your mother for her strong spirit and I believe this is what will carry her through anything.
As everyone else is tellng you, nutrition is the key. If you have to use sugar to boost the calories, don't worry about it. Keep her strong and well hydrated. I would look at the PEG tube, too, if you haven't already. The throat will get very painful and swallowing may be more of a chore than she can handle for a period of time. Remind her she can bite on a bullet for pain but chronic pain does not help the body heal. Pain management has come a long way since WWII (and Viet Nam). With all that is available today, there is no reason a person should have to fight through pain.
Try not to worry and take it one day at a time. Good luck and best wishes for a speedy recovery for your mother.
Ed