James, vits, mins and antioxidants should help a bit even if the underlying cause is mercury. B12 injections are known to help with MS (can get on the NHS or privately). Cut out all junk food and bad fats, substituting good quality omega 3 and 6 fats and lecithin (for fat metabolism). Even cutting out dairy and gluten can help some people.
If your mum is able to go through with Amalgam removal seriously look into Andy Cutlers (www.noamalgam.com) ideas for (POST Amalgam removal) mercury detox. Get hold of his book Amalgam Illness - click on link at top of this page.
Personally I dont think its ever too late to make significant health improvements whatever the diagnosis or prognosis.