85-year-old wife with late middle-stage AD is cold constantly. Any ideas on how to keep her comfortable?

This is the case for my partner, 82. He is always cold. Step one of course is your basic blood panel; you don't want to learn too late that you are dealing with thyroid problems or other issues that can be addressed medically. After that there are several things to consider.

Those who are more cold are often less active. Activity does indeed WARM THE BLOOD so to speak.
Those who are more cold often lack a body fat layer. Yup, our fat keeps us warm. That's why, in the Donner Party, the survivors were mostly women, who have a better fat pad overall.

In daily life and for comfort a knit cap keeps body heat in and long underwear is the oldy but goodie for body warmth.

Wish you the best. First stop is the MD office.

My late husband was cold year round, and we live in NC where it's warm to hot 8 months out of the year. He would wear either flannel pajama pants or sweat pants, along with a sweatshirt hoodie all year long, with the hood up around his head of course.
When he was able to sit in the recliner, I would put a heated throw over him, and when he became bedridden, I just made sure that he had plenty of blankets on to keep him warm.
Because they say that heat escapes from our heads, perhaps try keeping a hoodie up over her head, or some kind of winter ski type hat on her head to keep the warmth in, along with the other things you're doing.
I wish you the best in keeping her warm.

My mother was also always cold. The heat was set to 80°...in August...in Southern California! How my dad didn't die from being cooked alive I'll never know.

I've found that the best way to warm up is to have something warm between my thighs (a heating pad or hot water bottle) or on my lower stomach. When I lived in snow country, I'd buy those self-heating stick-on pads for muscle pain and put one on my lower abdomen or inner thigh. Those lasted 12 hours and were lifesavers.

Check with her doctor before employing any of those treatments, though, because older skin could be more sensitive to burns.

Just a thought, have her Thyroid checked. It controls body temps.

The skin on my entire back was chronically burned from laying on the lowest setting of a heating pad to treat bone cancer pain that responded to nothing but heat. My son bought me a heated padded vest from Amazon that's battery powered and rechargeable. It's worn over a shirt so there's no risk of getting burned. That puppy helped me more than anything and can be added to your wife's outfit quite easily. There are 3 heat settings on the vest, according to needs.

https://www.amazon.com/Foiueyga-Men%EF%BC%8CHeated-Included-Electric-Lightweight/dp/B0B59CS97N/ref=sr_1_14_sspa?crid=1DKKAPAVI6AV5&keywords=heated+rechargeable+vest&qid=1680348445&sprefix=Heated+rech%2Caps%2C257&sr=8-14-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEyRllCN0syRFVRUjVTJmVuY3J5cHRlZElkPUEwNDYxMjI2SVdLMkJFTlZCSERPJmVuY3J5cHRlZEFkSWQ9QTA4ODY3ODkyUzFYSE5EWEEwVlZKJndpZGdldE5hbWU9c3BfbXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

Individual hand warmers which are also rechargeable are great for keeping in pockets. They are small and look like a little plastic computer mouse.

https://www.amazon.com/Rechargeable-Electric-Reusable-Handwarmers-Christmas/dp/B0BF56MPT5/ref=sr_1_4_sspa?crid=2M0AQR2MMQ66I&keywords=heated%2Brechargeable%2Bhand%2Bwarmers&qid=1680348712&sprefix=heated%2Brechargeable%2Bhand%2Bwarmers%2Caps%2C244&sr=8-4-spons&smid=AEV7X0DGVZSP9&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzVkZCRTM0RVkwMDNLJmVuY3J5cHRlZElkPUEwMTc5MDQ3MVdRS1hJTjZEWjkwNCZlbmNyeXB0ZWRBZElkPUEwNjA1MTYyVE1TUTJUSjhLMUNVJndpZGdldE5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ&th=1

Oh my. My mom wears up to 5 layers. Some days 2 winter coats in her assisted living. I can wear a light sweater to visit. Lewy Body dementia includes temperature body issues. It is a tough problem. I have found no solution except a chair blanket…and learning to ignore all the layers. Be very careful of electric blankets..elderly are burned easily!!! An unsafe idea.

There is a brand of under garments tees, long underwear called " cuddl duds". They are designed for winter weather.
They are heavenly and may make a huge difference for her.

A hat keeps the body warmer if you can get get her to wear a little knitted beanie.

I was put on two blood thinners last year and had a lot of trouble keeping warm through the winter.

Fingerless gloves?

Microwave heatable rice packs. They provide a 'moist' heat for hours once warmed in the microwave. (They can get very hot, so experiment on yourself first to see what amount of time gives the right warmth. I start out with 90 seconds). When it cools, simply pop it back in the microwave for another 'charge'. You would need to find someone that can sew, but they are easy to make. I've made numerous ones for friends and family. Simply sew a pocket to hold the rice...either in a tube shape or a small pillow, fill with white rice from the grocery store. The ones I have made are about 18" long and 4 inches wide for use around your neck. The tube should be filled so that it is flexible enough to lay flat and conform to your neck. I use muslin to make the casing. Then I make a removable covering that can be washed. I secure the ends with velcro. Rice packs seem to last a long time. I have one that I have used for several years. Simply place it on her lap when she's sitting...then cover her with a lap blanket. She will stay toasty for an hour or longer. Good luck.

My solution was an electric blanket that I put over my mother when she was up and sitting in her chair.

The brand I bought is UGG and it's absolutely luscious. Soft, very thick, machine washable and doesn't get crazy hot.

Put the blanket in the dryer to warm up before placing it on your loved one.
Can easily put most clothing and blankets in the dryer- even towels for when getting out of the shower.
granted- the warmth from the dryer quickly recedes, but your loved one will love the initial warmth.