First of all, keep in mind that I am unable to diagnose you because I am unable to examine you, this forum is for educational purposes.
I am sorry to hear about your mothers cancer diagnoses, I can not tell you from the data that you provided where the pain is coming from, but I can outline several possibilities. It is possible to have pain in the shoulder area after an abdominal procedure due to refered pain (the area under the diaphram is connected to nerves so that an insult in that area can cause pain that is felt in the shoulder). One common reason that this happens is an infection/tumor in the ovary. I would make sure that the diaphram/upper abdomen was explored/imaged for any sign of infection/additional cancer due to your current pain. Another possibility is a brachial plexus injury sustained due to positioning during surgery. In abdominal surgeries the arms are often placed over the head, and under anesthesia/paralysis more pressure than intended can be placed on nerves. The fact that the pain seems to shoot down her arm and into the fingers suggest a brachial plexus injury is possible. For this possibility I would suggest an EMG (nerve muscle test) to evaluate the brachial plexus. The final possibilty is that your pain may be due to a metastatic focus of tumor that is tracking along the nerves, in the spine, in the brain or in the shoulder itself. You have mentioned some MRIs etc, but if you have not have a brain, entire spine and chest/arm MRI with Contrast, this may be helpful. I suspect medications for neuropathic pain such as neurontin, elavil, lyrica, cymbalta etc will be most effective.
I hope this has been helpful.
I asked this cos I've got a condition called complex regional pain syndrome (aka reflex sympathetic dystrophy) which can start up after an injury or after surgery.It's a condition of the sympathetic nervous system and means that the pain receptors don't turn off like they normally would after an injury or surgery has healed (basically your brain thinks that there is still something wrong causing it to hurt along with some or all of the other symptoms aswell.
I hope that your Mum manages to get it sorted out though
I will always be in pain,therefore at 54,all body parts are very important.
To amputate is there still the possibility the brain will still generate the pain such as service men and/or women and car accident victims who have lost a limb?
Your mother is stronger than she thinks and be sure and keep enforcing that. For me, just to know that it is not just in my head, that this is a bonafide medical problem is like the difference between night and day.