Any suggestions for an amazing mattress for Mom who is increasingly bed bound at home with hospice care?

Spring Air edge support mattress dual summer/winter pillow top.

My spouse under hospuce. Bed frame and mattress provided
Check with your hospuce nurse. It makes a bug difference on the comfort level.

As my spouse ages and his Parkinson Disease progresses, I have found the perfect item.

I have a blow up mattress pad like the ones used in the Hospital in our that keeps the pressure evenly distributed, I can pump it up or decrease the air.
it works wonders, as well as the Honey Comb gel Seat covers for his Wheelchairs seat and his walker, the car and kitchen chair when he can get up.

She needs a low air loss mattress, a real one. The one I got for my dad was on ebay. I believe the company that makes it is called Sizewise. There's also a company I believe called Agiliti that sells them. This is not the same as an alternating pressure mattress. The true low air loss mattress prevents pressure injuries. It has tiny little holes everywhere that you can't see, but it circulates air around the patient to keep their skin dry. Moisture is the enemy when it comes to wounds!

The hospice shd be helping you

If your mom is on hospice, under Medicare, they provide a hospital bed and inflatable mattress to prevent pressure wounds. My husband had such and my brother-in-law as well. They provided things to keep them comfortable. Check into that.

I have a Temper Pedic, but when I was in the hospital recently, I woke up on one of those pressure-compensating mattress toppers, and it was way more comfy than my expensive bed!

Your Brother says a new mattress? Then let him pay for it.
Meanwhile, get some Flexeril for Mom's pulled muscle in her back, it works great! What I used recently.

Is she on a regular bed? Or does she have a hospital bed?

A hospital bed can make a huge difference! Because the back can be raised, the legs can be raised, and get one with adjustable bed height, which makes it easier for care providers as well as setting just the right height for mom to get in and out.
Also, just changing the position a little bit a few times a day, to shift the pressure points.
I feel like mattress choices are subjective. A firm mattress is best for some, while others will benefit from something softer, with more give.
Unless she has a really bad mattress now, I don't think a change in mattress will make much difference. If she says this mattress is uncomfortable, then I would try getting the opposite. For instance, if she is uncomfortable with an overly soft mattress, try something firmer, or with spring support. If it is too hard, then try something softer.
A pulled muscle in her back is going to take some time to heal. Just finding different positions for her body, moving her gently, and taking some ibuprofen will help.

Whether or not a mattress is comfortable or not is a very subjective measurement.

We have a king sized Select Comfort mattress where each side's firmness is separately controlled. My husband has his at 50 (which feels like sleeping on concrete to me) and mine is 25 (and he says it feels like sleeping on a marshmallow but not in a good way).

Your Mom pulling a muscle in her lower back probably has nothing to do with her mattress. And healing the pulled muscle (or preventing future pulls) will also probably have nothing to do with a new expensive mattress.

Has she actually complained about the mattress? Maybe some PT or a gentle massage would be a better solution.

If I had stupid amounts of money I would buy one of those electronic adjustable beds so I raise the head, sit up, bend at the knee and raise/low it vertically. I looked into one for my Mom and they cost a ton. My Mom prefers to sleep in her recliner because it conforms to her body better.

I love my Purple mattress! One of the great things about it was that it had a 90 day return policy. There are a lot of mattress companies out there that offer that now. I suggest going with one that does just in case what you find isn't the right fit.