Bike with coaster brake and gears for teenager with disabilities
If I understood your question, you may want to look into internally geared hubs, it seems they are exactly what you are looking for.
I thought they were fairly popular, so I would be surprised if you really have not come across these - so I am sorry if I missed the point.
A landmark feature of gear hubs is that you can change gear while stationary - it sounds like this could be beneficial to you too.
Have you spoken to someone in a good bike shop? I would imagine it should be easy to find one where you can test ride a bike with a gear hub.
Alternatively, you could google something like &quot;bike with sturmey archer gears&quot;. You mentioned &quot;cruiser bike&quot; - this seems to be a fairly broad term, if you can add the brand and model of the current bike (or a photo of it), people on here may be able to suggest similar alternatives, and I think it would be within the stack exchange rules about product recommendation.
If all else fails, you could probably get in touch with a bike manufacturer that sells bikes with these, to see if they can assist with getting a test ride? This almost sounds like a perfect story for a bike maker's Public Relations. Good luck and I'd love to hear how you got on with this!
Daniel R Hicks commented that gear hubs with a coaster brake are rare. Sturmey Archer website lists a few, you can't filter but you can search it for &quot;coaster&quot;. I believe the company had some difficulty but seems to still be in business.