The society has liquidated a lot of assets recently they are not poor. The legal route coupled with media attention is the best way to go. Try to see if a firm will take on your case pro bono or they will take the fees from the society. However be aware the society plays dirty and uses every legal trick to stop complainants getting legal support.
Do you have any paperwork for this conditional loan? If there is paperwork then a lawyer can write to them quoting articles in the document in outlining legal action. He would also be able to advise why there is a reason why they do not feel that they need to return the money.
I would also take it up with your local political representative make a fuss on three fronts. They are relying on your mother giving up anf going away. On no account should you do that. To keep hold of her money is immoral.