My 99 does the same thing with the soup. They also drink soup in one big bowl. She wasn't happy when I got a separate bowl for me and my hubby and complained. I didn't explain and just ignored her. Now I have separate bowl of soup (he he).
I don't have the rice problem. If I were you, I would bring my rice or get a separate rice cooker (if living together) and tell her I don't "eat other people's siliva".
Best way to deal with trouble 99 - don't explain, don't argue, just ignore her. Some old people are just bored, they like to pick fights to add excitement to their lives or to stimulate their brains.
My 62 will start an ordinary topic and then turn it into an argument, I usually say 'yes, you are right, I must have mistaken.' and change the subject to today's weather.