Mom in hospice for congestive heart failure. She's also had breast cancer and recently developed a rash. What could be the reason for this?

It could possibly be Shingles, after she has had cancer.
I really don't know, but it is my understanding that hospice is about comfort care. One can barely do that if there is no diagnosis by a doctor.
There is a hospice doctor, right? Not sure, but you can still have her seen by outside of hospice doctors? Others will know better than me.

From what I read, shingles can be very painful. It also may be communicable, I would have to look that up. (the chicken pox is communicable to immunosuppressed persons if they have not had chicken pox). That was a very simplified explanation, sorry.

In light of people 'graduating' from hospice, after improvement, not dying when expected, I would look into a diagnosis, then go from there since hospice does not do 'life saving/life sustaining"
treatments.

Keep in mind that a person can go for treatment instead of having hospice. I am not recommending you do that, every case is different.

You say that the CHF is "no longer the main problem" but you observe edema. The edema is likely a result of right sided heart failure. If there is shortness of breath then there is also left failure from fluid in lungs. If Mom is developing rashes it may be due to medications or any other reason. Dermatologists are great for saying "Who knows, anything but an exact science". Your hospice nurses are caring for Mom and are the ones most likely to see what it "may" be. You mention uremia, so you are well informed. Opioids, uremic acid, almost ANYTHING can cause a rash. Sorry; it does so complicate when there is already ENOUGH suffering going on. I hope Mom can move to in facility care soon.

My first thought when I read this was perhaps your mom has developed shingles. My husband was under Hospice care in our home for 22 months and he also had developed a rash on his chest and back. They(hospice) asked me if I thought it could be shingles. I answered, how would I know. I've never seen them or had them. They decided to treat his rash with some kind of powder and cream that was supposed to help with a yeast type infection. It didn't help, but my husband had so many other issues and the rash thankfully didn't seem to bother him, so we left well enough alone. It's been my experience with Hospice that they tend to do much better with those patients that die in a more timely manner, unlike my husband and your mom.
And as far as her kidneys possibly shutting down, if they were, you would notice your mom's pee being much less and dark in color.
All of this is tough on you and your mom, and I am so sorry you are having to deal with all this. Just please enjoy whatever time you have left with your mom, and make sure that Hospice is doing everything in their power to help you. ( I had to stay on top of them ALL the time). Hopefully your hospice agency is better than my husbands was. God bless you.