Hi,
I have very limited experience of Disney resorts as we have only ever stayed on-site once.
Although we stay at Polynesian Resort next year.
We had a full tour around SSR in 2006 - personally I wouldn't choose to stay there as the resort is quite spread out and a fair bit of walking may be required, although naturally that could depend on which building you are located.
Our son Joshua is autistic and whilst fully mobile has difficulty walking long distances so we actually bring a child's wheelchair with us.
To be honest we encourage him to walk as much as we can and only really use his chair at the parks (so the circumstances are different I know)....but didn't fancy SSR as it can be a fair walk to the hub of the resort.
We chose the Poly because it is close to Magic Kingdom, on the monorail route with the likes of the Grand Floridian nearby plus whilst there are different accomodation buildings at the resort itself it isn't as spread out as some of the
DVC resorts such as SSR and Old Key West.
I am sure your mum will like the theming, there are some nice restaurants and shops there too.
Being on the monorail route could be a big advantage to you I feel.
Staying somewhere like GF, Poly or Contemporary would suit your circumstances.
The other option is a standard hotel type . 1 building, easier access to everything and less walking involved.