Mom and I have been fighting about 2 weeks about going back to the doctor. She is now under 100 pounds as well. Her insides are working very well, so she she does not the same problems as your mom does.
My mom suffers from unbelieveable pain from 8 broken backs and 2 torn rotater cuffs and very old knees.
I don't know about your mom, but my mom is of very sound mind. I could never convience a judge that she needed a care taker. She is just (at least I thought this till I read your troubles) the most stubborn woman on earth.
She is also afraid of doctors. They intimidate her. Her main doctor sees her every 4 months......I really like him and he is married to a high school friend of mine....and he has 10 kids. She does not like him....mostly because he does not sit down with her for an hour and chat. (Medicare makes it so the doctors have to see as many patients an hour as possible). He says things like "your doing great considering your age" (makes her really mad).
The knee doctor pretty much blows her off.....(we have had her knee xrays looked at by 4 different doctors and they all say the same thing....looks like knees of a 40 year old).....no one can tell her why they hurt so much.
The back doctor is worthless......
The rotator cuff doctor (this is a new injury ... 6 months ago) my mom really liked him.....I mean really liked him.
I tell you all of this, because that is my key....the fact that she likes him and he does not intimidate her. So TODAY finally, she told me I can make an appointment with him and she is going to tell him all the other doctors are hogwash.....(she will most likely use a word I can not use here

)
Pain management seems to be all they will offer.......and pain management does not work very well and I know she is hurting. She can not even put curlers in her hair anymore. She never goes and gets her hair done anymore.
She has done the shots of cortizone....they last about 3 weeks....and she can only have one every 4 months....
You are in a very tough spot. If she were my mom, I would drag her.....otherwise she will die and you will feel terrible.....at least if you give your best shot, you can be a peace with yourself.
Maybe try a new doctor....or at least talk to your mom about why she won't go.... (my mom won't because she says they are all ba****ds)....my kids yell at her to stop talking like that
Sorry to be so long winded.....you are in a very tough spot......just do your best so you can be at peace with yourself, whatever you find that is....