Interesting concept, and one I have fantasized about in the past. You have seen most of the problems, but why worry about logic? If you are uncomfortable with abuse, just develop a society where self defense is totally legal, even from a sub.

Robert A. Heinlein did this in a few societies that he wrote about, people were armed, there was a strict code of conduct that was enforced, not by the government, but by the people. This might be a little tricky to make work, but it would be feasible.

As for having an incentive to be a sub, wht not just do what John Norman did in Gor? Either a society, or a simple genetic bias toward a dominant sex. Or, if you want a gender neutral world, two races that are similar in appearance, but have a significant diffeance that makes one naturally submissive, and the other dominant. Actually, this might be the best way to go, it gives you a wide randge of subs to play with. Couple it with a society that does not tolerate the physical abuse you want to avoid, and the societal impulse that allows any individual the right to make that call, and you have a world that could be perfect.

Of course, there is still the problem of lying. But a false report could land the one that lies in a situation that is much worse than the one he or she wants to avoid. Those are just the details. Let me know if you like this, but I do reserve the right to use the idea myself.