I’ve been thinking about your control set up. From your description of your condition, you can only tighten and relax your biceps for arm movement. For example, you could tighten your biceps to curl your forearm up at the elbow, but to return the forearm to the original position you can only relax that muscle and let the arm drop by its own weight. You could not force the forearm back using your triceps.
Is this accurate?
I’m wondering because your original post mentioned a push-pull throttle-brake set up, but it sounds like your arm could only accomplish the pull and not the push (or visa versa)?
Maybe not the end of the world, as we could just make it spring loaded to get force in the direction opposite to the direction in which your bicep can move your arm.
The steering would be a bigger problem. Would you be able to use muscle power to turn a wheel in both directions with one arm? The guy in the You Tube video had pretty good arm motion and appeared to be able to use his triceps.