--that in some people even blood sugar readings in the prediabetic range can cause neuropathy.

But I think the much more obvious aspect here is vitamin deficiency. She had gastric bypass surgery in the past--that is absolutely associated with difficulties absorbing vitamins, especially the B-complex ones and especially B12.

And B12 deficiency (that level of 387 you mentioned was much too low, despite typical American lab ranges; in Europe and Japan the lower range limits usually start around 500-550--we just seem to take a long time to catch up to more recent clinical thinking) is not only associated with peripheral neuropathy, but with multiple systems degeneration that can include cognitive/memory issues and motor problems. In fact, in gerentological circles it is established that among elderly people with cognitive/memory issues one really ought to check their vitamin levels first before assuming dementia or Alzheimer's; B12 and other B-vitamin deficiency is quite common as people get older as the intrinsic factor needed to break out cobalamin from foodstuffs in the stomach significantly declines with age (with or without alcohol consumption).

I'd see if you can get your mother's doctors to monitor her vitamin levels regularly, and to continue her physical therapy. Hopefully she will improve over time, but neurological damage takes a long time to heal, if it can, and the recovery is often patchy and incomplete.