How do you deal with an 80-year-old-woman who will not shower or bathe?

Have you read the excellent article on this site about this topic? https://www.agingcare.com/articles/elderly-parents-who-wont-shower-or-change-clothes-133877.htm
Your mother may not have dementia, but she apparently has some kind of impairment in her reasoning, at least on this topic! With the heat and the chair you are obviously trying to overcome her fears but you must not have hit the right hot button yet! What does she say when you discuss this with her?

Are you in the home with her or does she live alone if she lives alone I would call social services and tell them she is not taking care of herself and they need to step in if she can get downstairs and eat and smoke all day she should be able to wash herself or let someone help-have you reported this to her doctor-she is choosing an unhealthy lifestyle especailly being a diabetic-not being clean can lead to a host of skin problems like ulcers that might not heal and massive infections if she is being stuborn to gain attention which can be true-by going along with her unhealthy lifestyle you are enabling her-if you do live with her tell her you will have to leave this is unhealthy for you also and the smell must be unbearable-good luck but you need to take action.