I know exactly what you are going through. My Mom has to let me know from time to time that she is tired of me always telling her what to do (what to eat, what to drink, what questions to ask the onc., etc.). I think the great thing is that you and your Mom had a discussion about it so that you both could understande each other's perspective.
There is an Italian pasa called "pastina." It's very tiny stars and a couple of spoonfuls in a cup of broth can be very gentle on a stomach. I was raised on this stuff--for the stomach flu it was often the only or first thing we could keep down. My Mom has used it for several of her friends who have undergone chemo over the years and they reported it was often the only things their stomachs would tolerate. You might give it a try. It is made by Ronzoni and is in the pasta isle (sometimes it can be found with the babyfood because it is also used as a first food for babies).
Susan