I possibly would reverse a couple of your steps but if you have a plan, go for it. Even though you experienced seeing the bike start without the twist grip cable plugged in, I would not count on that being an error you can duplicate at will. Maybe yes, maybe no. It makes no sense to me that the bike would even start with the "summing +5 volts" missing. Yes, I believe you when you say it happened, but looking at it software wise it makes no sense at all. It really indicates to me that there may be a loose screw in the error detection program but who knows. Think about it,, It's one thing if the bike was already running, the cable happen to break and the engine kept running at idle speed. It's another thing if the cable was cut and the bike would still be allowed to start. To me, that would be lunacy. These types of decisions are part of a written program. Written by who is what scares me.
I would verify the twist grip pots first, get the summing voltage present and corrected. That must be corrected regardless if it fixes the injectors or not. If it does fix the injectors, then you don't need to swap throttle bodies. If it doesn't fix the injector problem, just proceed.
Either way I am curious to see what you find. You do have an interesting problem.