This is a tough situation. A patient with dementia maintains the right to make his or her own decisions (including limiting, refusing, or stop medical treatments) as long as he or she has legal capacity. Power of attorney does not give the agent the authority to override the principal's decision-making until the person with dementia no longer has legal capacity. For more info: https://www.alz.org/national/documents/brochure_endoflifedecisions.pdf
Of course, you are in a very difficult bind with little support for yourself. It is very important for your health to be a caregiver to your Mother with the least stress possible.
If your mom has dementia you can make healthcare decisions - and other decisions - with a Power of Attorney very quickly and with little to no cost.
If your mother is able to converse with you, let her know you need her help to help her. She needs to authorize you, if she can, to grant you permission to be her POA..
If not, just call the doctor’s office and tell the doctor the information he or she needs to know...Providing the information does not violate the doctor patient relationship.
It allows the doctor to not have their hands tied.
First, god bless you for your care for your mother....sounds like she needs to be in a nursing home, She may be able to seek Medicaid if funds not available...in fact if she has not hit Medicare limits that may cove for a period long enough for you to have some rest and time to get a diagnostic help. I understand how your mother can wish to cross over - to rejoin her husband, regardless of any specific religious context.
Sounds like you have a challenge getting her to agree....tell her how exhausted you are and how you need her help in getting other care ... she can state to the care facility her limits on what they can do to txtend her life. Hope she has a Directive for doctors to follow. You can get advice on that from the web doctors, hospital....I believe. My be available in a simple form style.
Get some help, please