That is all fair enough and I like some of the ideas you have set out there.
The method I had in mind was to form the debate in a regimented structure:
1) The thread is created by the adjudicator/referee.
2) The person arguing FOR the motion has 1000 words to outline his/her argument.
3) The person arguing AGAINST the motion replies in kind.
4) Observers throw out questions, which are collated and offered to the two debaters by the adjudicator/referee.
5) The person arguing FOR the motion has 500 words to answer the questions and 200 more words to conclude his/her argument.
6) The person arguing AGAINST the motion has exactly the same afterwards.
7) The winner is decided by poll/popular consent according to who had the more 'persuasive' argument.
I was planning on having moderators keep the thread tidy by only allowing the debaters to post anything but questions in the middle.
---
Your plan is pretty good too but a back-and-forth argument could lurch off-topic/go off at a tangent very easily and I would not want that to happen.