As a caregiver to my husband, who has dementia, I have solved the problem of bathing and shampooing, for myself at least, and I hope that how I am handling it might be of some use to other caregivers on this forum.
What I do now, as he has progressed where he doesn't want to sit in that chair in the bathtub anymore is this. I'll just saunter into the bathroom one morning while he is washing his face and hands, and I'll say, "Good morning! This is the morning when we wash your torso. And I have a clean undershirt here and a clean shirt, isn't that great?" Then he'll smile and say, "Yes." So then it just takes a matter of minutes to help him off with his shirt and undershirt and then I wet a washcloth and soap it up and give it to him and he washes his torso, under his arms, his arms, etc., and then I do his back, and I'm singing a catchy song at the same time. He loves this.
Then I dry him off with a towel, and I hand him the deodorant, and then VOILA! Done! Easy as pie! Remember, there is no law of the land that says you MUST give your patient a full bath all at once. No law at all! You are free to improvise, which is what I have done.
Then either the next day or a couple of days later, while he's watching TV, I bring a large bowl of warm water, strip off his socks and shoes/slippers, and place his feet in the water. I push up his pants, soap and rinse his thighs, legs, and then do a real job on his feet, dry them, clip his toe nails, followed by a fancy pedicure, including massage of his legs and feet with a foot cream. He loves it, and it's really a relaxing experience for me. The whole time he's watching TV.
Now we're all done with the bottom half of him (his privates are something he keeps washed and clean every day. Once in a while, he acts confused, and then I step in to help in out in that area; no sweat.)
Then a couple of days later, I shampoo his hair while he's in the bathroom washing his hands. I just step up to the plate, say "Guess what? Time for one of my fancy shampoos," and he says, "Okay," and that takes a matter of minutes, then rub with a towel and we're all done.
So I have found by keeping him clean and sweet smelling in bits an pieces takes all the anxiety away from both of us.