I re read what you had written re your wife's scoliosis and thinking she couldn't lean forward re riding. I have some suggestions or ideas about that as I had to ride in a really upright position for awhile.

First I'll mention my recumbent trike which I purchased in Florida while living there. I went to a bike store that had a number of them in stock so I could at least check them out before purchase. I found one I thought would work for me and also purchased this extra firm cushion to use against the back of the seat that I still have since 2004. It's still firm (is about 14 inches high and 12 inches wide with a thickness of an inch and half to two inches and has protective covering that is washable an laces up in the back)! It really helped me greatly back then. In fact my father used it to sit in a regular dining room table chair when his spine became so extremely prominent from "just being skinny" (he was in his late 80's). So that's something you can add with a seat that has a backing like a recumbent and sit even more upright.

My beach cruiser allows me to sit very upright especially with handlebar that allow my posture to not be bent over or leaning forward.

Good luck with your bike search!