I would solve this by everyone keeping their own accounts so personal stuff like shopping lists stay separate. Although all devices will be separate they will now use a single home network. Not a problem. Mom should keep her Echo somewhere most convenient for her, and having the same wake word as your devices will likely not be a problem if it is far enough away from all your devices, but if it is, Mom's wake word should be unique and different than any of your wake words.
As far as sharing stuff, that is a question to pose to Echo support.