I am wondering whether SS have assessed your mum over the phone. They did this to my mum and (no surprise) she told them that she was doing all her own shopping, cooking, housework etc, and even though one look at her and her home would have told you otherwise, because it was over the phone mums statements were taken at face value. I wasnt told this was happening and only found out about it later.
Have you contacted SS yourself and told them about the fires, aggression etc? You mum will not have said anything about this as she will be unaware that it is happening.