I am so sorry you're in this situation. As far as the hospitalization for her fall, if she has homeowner's insurance, it should cover her medical bills due to the injury if her health insurance won't pay for it.
I'm not sure why her financial situation will fall to you. Is she broke? She needs to pay her own bills under your guidance. You are not legally responsible for her financial obligations. If you are taking care of her, and paying for things like food, toiletries, etc, can you pay for it with her credit card? If she is in debt when she dies, that will all go away.
Since she won't sign the power of attorney documents due to the pain medication, can you at least ask her to set up her banking online so you can use her login to electronically pay bills w/o her signature?
While she is still lucid, get the ball rolling for Social Security and Medicaid. I think she needs to be considered indigent to get Medicaid. There is a lot of paperwork involved, so the sooner you can get her to sign everything the better.
If she becomes incapacitated, you can file legal papers for establish a conservatorship for your MIL. This is also a fairly involved process, and requires the help of an attorney. My guess is that your MIL will probably die before you can establish a conservatorship. If and when she dies, your husband will probably have to be the executor of her estate.
One of the previous posters mentioned hospice. This sounds like a great idea. They will be able to help your family as well as your MIL.
Please take care of yourself throughout all of this. Good luck to you. It's an impossible situation.