I love this idea of systematic consensus building. Why not just use the polls feature as is and simply set up the question this way? The ability for people to add questions may not be that important, or could be dealt with ahead of time. So you could start a topic saying “let’s do a poll! What should the answer options be?” Then set up the poll after collecting answers.
Incidentally, I do often run into trouble with polls which can’t be changed after the poll is started. As we use it, we discover issues with the questions or need to add more answers. Is there no way around this limitation?