I used Karabiner for a month after getting hit by a car and breaking my right wrist. I couldn't use that hand at all whilst it healed.
I had Karabiner remap my keyboard to half-qwerty, so that I could type entirely using my left hand on the left side of the keyboard only. A modifier key would switch it into "mirror mode" so that all the left-side keys would be remapped to right-side keys. Fun stuff!
The biggest obstacle was the swelling in the wrist after surgery. I had to keep it elevated for quite a long time, and it would throb and be painful when I lowered it to the keyboard. That passed gradually over 2 months. The finger dexterity never left, the moments I held my hand there I could type at ~80% speed without problems from early on.
With just my left hand, I could only manage more like 20%, which felt very very slow to me. The bottleneck with half-querty was the modifier key, which I had to hit very often to switch layouts. If you could move that to something else, that layout could speed up much more.
Had I been out of action much longer, I might have tried some kind of custom one-handed chording setup instead.
I'm adding my comment here to "pile on" and say that I'm very interested in the answers to these questions, too.
I tried to learn a one handed keyboard back in college (Matias Half Keyboard) just to see if there was any benefit to always having one hand on the kb and one on the mouse.
I never did find out; I gave up learning how to type with one hand after a month and sent the keyboard back.
I had a bike accident which prevented me from typing with my left hand for a couple weeks (dislocated shoulder, broken finger). I wish this were around at the time! I had looked into physical half-qwerty keyboards, but I couldn't afford one.
I ended up hobbling together a simple app that switched two full system key layouts while holding the spacebar, more or less mimicking the half-qwerty behavior. It was my first Mac app, and it was pretty crashy, but it kept me working for those couple weeks.
I wanted to open source it, but ceased work when I was able to type with both hands again, because I knew half-qwerty was patented and didn't want to take that risk.
I had Karabiner remap my keyboard to half-qwerty, so that I could type entirely using my left hand on the left side of the keyboard only. A modifier key would switch it into "mirror mode" so that all the left-side keys would be remapped to right-side keys. Fun stuff!