I agree with the above posts. Are the doctors she is seeing all communicating with each other? If not, they should so that everyone, including you and your mother are on the same page and the same plan of care.
Just to clarify above, not everyone over the age of 40 gets diverticular disease. Is she on a special diet? Did they check her for Crohn's disease or H. pylori (an infection)? Does she smoke? The fact that she has lupus indicates that she has an auto-immune disease, and Crohn's is also auto-immune associated with diarrhea and weight loss. But you state that she does not have diarrhea.
Where is her E.coli located (what part of her body other than the GI tract) or is it a different strain than the normal? Why aren't the doctor's treating it? Has she seen an Infectious Disease doctor or a GI doctor?
Your mother in law has already been diagnosed as having carpal tunnel problems, and this is the reason why she is experiencing pain, numbness and/or pins and needles in her hands. She should be wearing wrist support splints and not using her hands.
As your mother-in-law is experiencing problems in her feet and legs, this can be due to the lower back spinal problem. This is referred pain. There are also tendons in the feet and they can get inflamed or tear.
The instability can be due to degeneration of the spine, but as your mother-in-law has low blood pressure, this will cause her to feel unstable too.
Most people over the age of 40 will have diverticular disease. It is important to have a healthy and well balanced diet with plenty of fresh fruit and vegetables with plenty of water, so as not to get dehydrated or constipated to avoid diverticulitis. As your mother in law does suffer from GERD, it is best to avoid all fried and fatty foods, highly spiced foods and processed foods. She should eat small amounts of food more often and not eat too much all in one go. After a meal it is best to sit upright and not lie down straight after eating anything.
Anxiety can make any illness much worse, so it is important that your mother-in-law learns to relax. If she does become anxious, get her to breathe in slowly through her nose and then slowly to breathe out through her mouth.
You mention that your mother-in-law is going to have some more blood tests done and you will have to wait and see what the results of those will be.
I think I would have her see either an endocrinologist or a neurologist, some of her symptoms would appear to be neurology related.