Delusions that are getting so much worse. Any advice?

I doubt your mom's competency. Is she really doing her checkbook but still does not believe that she has spent $4600.00 on mattresses?
Anyway...
buy a mattress topper. They come in soft to firm.
(why did you not take the mattress if it is newer than the one you have currently? Or for a spare room?)
I would begin to curtail shopping with her.
Might want to review the "rules" of the community and see what happens when money is low to gone. Will they accept Medicaid if she needs skilled nursing?

Purchase a 3" Memory Foam mattress pad for whichever mattress will work.
I got mine from Costco.

When a neighbor with Parkinson's went through this stage of changing mattresses every month, he could afford it.

So, maybe stop asking how the mattress feels.
No one wants a used mattress, not the least of reasons, it is against Health Department regulations to sell a used mattress.

Try pain medications at night.
Try pillows.
Ice packs if she will tolerate them.
Make the sheets and covers very soft and lightweight.
Try feather beds on top of the mattress.

Try 20 mattresses....
Once upon a time there was a prince who wanted to find a princess, but she would have to be a real princess. So he traveled all around the world to find one, but there was always something wrong. There were princesses enough, but he could never be sure that they were real ones. There was always something about them that was not quite right. So he came home again and was sad, for he so much wanted to have a real princess.
One evening there was a terrible storm. It thundered and lightninged! The rain poured down! It was horrible! Then there was a knock at the city gate, and the old king went out to open it.
A princess was standing outside. But my goodness, how she looked from the rain and the weather! Water ran down from her hair and her clothes. It ran into the toes of her shoes and out at the heels. And yet she said that she was a real princess.
"Well, we shall soon find that out," thought the old queen. But she said nothing, went into the bedroom, took off all the bedding and laid a pea on the bottom of the bed. Then she took twenty mattresses and laid them on the pea, and then twenty featherbeds of eiderdown on top of the mattresses.
That was where the princess was to sleep for the night. In the morning she was asked how she had slept.
"Oh, horribly!" she said. "I hardly closed my eyes all night. Goodness knows what there was in the bed! I was lying on something hard, so that I am black and blue all over my body. It is horrible!"
Now they could see that she was a real princess, because she had felt the pea right through the twenty mattresses and the twenty featherbeds. Nobody but a real princess could be that sensitive.
So the prince took her for his wife, because now he knew that he had a real princess. And the pea was put in the art gallery where it can still be seen, unless someone has taken it.

Would you please stop entertaining your mother's behavior. No more mattresses that's it if she doesnt like it too bad.

You can stop her games anytime by refusing to play. It's time mom was ignored and left to her own devices. Let the chips fall where they may with that.

At this rate the stress will kill you and she will still be alive. You and hubby are too old yourselves to put up with this nonsense.

In reflecting back on my own father who was a very independent man his entire life, at 83 when he became ill, he was determined to stay the course and it was hard for him to admit he needed help from the family but outside help as well. To lose one's sense of being independent can be frightening as if they're losing themselves in the process. So yes, we experienced from time to time exactly what you're going through and it's not easy and can take an emotional toll. I sought a therapist for myself to keep myself sane! There's a condition known as "Sundowners" and it does occur when the sun starts to set and for some especially with dementia causes a distinct personality change. They can become restless, argumentative, and confused! Is your MIL as you described like this all day, or have you noticed more problems in the early evening? Please take good care of yourself and not good caught up in the crazy times, something I know is easier said than done!

Mean people become mean old people. She is using you and will do so as long as you give your permission. Kudos to you for putting yourself and your husband first. Have a great getaway! I can tell your burden has already lifted just knowing you're soon out of this rut.

Just want to say, in my State anyway, selling a used bed is not allowed because its a health thing. I would never buy a mattress or any upholstered furniture from anyone. I worked for a bedding manufacture and any mattresses we took back because we replaced under warranty had to be burned.

Ditto to Sendhelp's suggestion of a foam topper -- but make sure to put a waterproof cover on it, in the event she becomes incontinent at some point.

Your methods used for toddlers won't work because she's not a small child with a brain that is learning and growing. She's an adult in aging decline who has a broken brain and is losing the functions of reason and logic. I highly recommend you learn about dementia by watching Teepa Snow videos on YouTube. You'll get insights on how to better engage with people with dementia so that interacts can be more peaceful and productive. That's what I did and it was totally helpful.

Your mother doesn't 'need' any sickness or ailment or dementia to be 'ready' for Assisted Living. She just needs to be tired of living alone and/or cooking/cleaning and ready to have others waiting on her and ready to assist her if and when she needs it. She needs to want to have entertainment available to her, music, movies, side trips, shopping, meals served in the dining room, and about 50 other services available to her if she'd like to take advantage of. And a doctor who comes to the AL to see her at least once a week, run tests, do blood work, etc. I don't know where the idea comes from that an elder has to be deathly sick or riddled with dementia to be 'ready' for AL, because in reality, by then it's too late for AL and it's time for Skilled Nursing or Memory Care if the dementia is advanced. AL is for functional seniors who want day camp and others to schmooze with and complain about things in general.

As far as the mattresses go, my mother was similar to yours. I bought her a new mattress and adjustable base which she hated. Which was unfortunate because that was the bed and mattress she was going to have, period. She could complain to whomever she wanted to complain to (in her Memory Care AL), and that was fine, plus she could complain to me and ask DH every week what he thought of her 'new bed'? And we'd tell her the same thing every time: that her new bed was lovely and just exactly what she needed. She would disagree, and then we'd change the subject. The bed was hers, and that was that.

She did the same thing with clothing, small appliances, decor items, furniture, you name it, she'd buy it and then insist it had to be returned. Even Oil of Olay face cream! She'd call the company directly & lie & say there was something 'wrong' with the jar, and they'd send her a coupon for a free jar! My father coddled her for decades, and would drive her back & forth to stores to return all the merchandise she'd buy and wind up hating, but once he stopped driving and they moved to my state, all that stopped on a dime b/c I refused to entertain her with it.

One time, when she lived in AL, she returned a pair of shoes I'd broken my back to buy for her, behind my back and unbeknownst to me. She had asked the bus driver at the ALF to drive her over to the shoe store so she could return the shoes, and he said okay. I wound up calling the shoe store a few weeks later to ask about a pair of shoes, and the owner told me she'd returned the pair I'd bought her. I nearly burst a blood vessel in my forehead when I heard that! I told him I was driving over to buy them AGAIN and to please mark NO RETURNS on the receipt, which he did. I showed up at mom's apartment with the shoes in tow and boy howdy, she was quite surprised to see me with the box in my hands. That was the last time she returned the shoes she was REQUIRED to wear to help her maintain her horrible balance issues. Sigh.

So I'm with sp on this. No mattress returns. No mattress toppers. Nothing. That's it. You can buy the woman a solid 24 karat gold mattress and she will find a problem with it, 100% guaranteed. That is what these women do: they find fault with what they purchase for entertainment purposes AND to drive us crazy. Those days are over now. Because we're no longer playing those games. It takes two to tango and if you're not playing, mother's dancing alone.

The bed she has is perfectly fine. And if she doesn't like it, she can sleep on the sofa. After a few nights of doing that, she'll find the mattress to be absolutely DIVINE.

And, FWIW, your mother is not 'competent', she's got dementia which only gets worse and worse as time goes by. The tantrums and nastiness only get worse and you'll see that as your need for medication increases! "Competent" is a definition that changes according to how you look at things. Start looking at this as your mother needs more help than you realize, and YOU need a break from all of her histrionics. It's time for AL!

Good luck!

$3600 tip?