Reading through your letter, I know that you are frustrated with your mother. Speaking from experience, I know it is difficult to help people who are having problems and then
refuse your help or advice.
Your mother actually sounds like she has chronic pain from her back surgeries. Chronic pain is pain that lasts for more than 6 months, and it can be very difficult to deal with. It is hard to treat with medications alone, as there is often a nerve pain component as well as musculoskeletal pain. It is also associated with higher rates of depression, which is sounds like your mother may have. This may be why she won't see her grandchildren or other family members. Think about it this way- it is hard to be fun and social when you are in excruciating pain.
What I would suggest is that your mother needs to see a pain specialist to get the help she needs. She may need an antidepressant, as well as other special medications besides painkillers, as she told your herself that they do not work anymore.