I am a big fan of the Cinqueterra in Italy, but I don't know if I would
recommend it to anyone with limited mobility. The main activity there is
walking/hiking. The towns are scattered up hill sides, and only one of the walks
between towns is a truly easy walk.
Also, there isn't all that much available in terms of shopping and music, or
art for that matter. This is an area of great natural beauty, but not full of
the cultural sites that one generally connects with visits to Italy.
And about the 15 steps and occasional use of a wheelchair: I think you ought
to reconsider the Cinqueterra. It is definitely not a place that is easy to
negotiate. Very hilly, very rustic, many steps. Even catching the train from
town to town requires walking up and down some steep stairs.
I hate to be a naysayer, but I am concerned that your mother may be VERY
inconvenienced by a trip to the Cinqueterra.
Why not extend the range of your trip to an opera-friendly town like Milan (nice
and flat) or Verona (much easier to negotiate)? If headed that way, you could
also visit the town of Sirmione on Lake Garda, and take in the castle. This town
is also fairly easy to negotiate, but very crowded on weekends.