You're completely right. None of this seems like an exact science. Her current treatment plan doesn't sound too off from the recommended CRT protocol. Perhaps they have their own statistics why they believe 2 chemo doses would be enough.
I have read that there's plans to have trials where they try to give lighter treatments to HPV+ oropharyngeal patients because they tend to respond well to radiation.

I doubt my mother's case qualifies for that though because her HPV status is still unconfirmed and she has matted nodes (does anyone know anything about that btw, when I found out those studies that showed people with matted nodes have high chances of distant metastasis it really crushed me).

In any case we should try and focus on nutrition now that her treatment starts sometime next week. I've already read your posts of minimum calorie and water goals during treatment. Thanks for that. Her oncologist also mentioned water intake is extremely important especially on chemo days.