As a former physical rehab nurse, I do have a couple of thoughts and maybe concerns. I would ask how well she does sailing on your boat. Can she control her body and maintain her position? In a wheelchair, her lower torso is pretty much locked into the seat of the chair, so it moves with her as if it were part of her body. On a bench seat in a boat, she can slide sideways and may also move front to back. That wont help her in remaining upright and in control of her position while she also had to tend the rudder and the sails. You mention that you built a lift to get her in and out of the boat, that's a big plus. How well does she manage with her own transfers?
My other thought would go to how she would handle herself if she did hit the water. I'm sure she would have her PFD on and even if unconscious, that should keep her face out of the water. But how well does she swim, with or without flotation?
Your original description sounds like one of the melonseed cats, very wide and stable and just one sail to manage. Another boat might be a mudhen. You could put a larger sized trolling motor on it for getting underway. A seat that somehow swivels or moves side to side would help her with control, I would just make sure that it is easy enough for her to get out of it in an emergency.
