Rashna, that guy (Jim), might as well have said "Allah knows best". The answers he has given are simply preaching to the choir. He is forgetting that he's facing someone who doesn't believe in islam. Although your approach is useful, I have often found that it might not work with some people. You have to remember that this guy has certain first premise on which he is building his arguments. His premise is the following:-
1 - God exists.
2- Mo is his messenger.
3 - The Quran is the word of god.
4 - The hadith are words and sayings of Mo.
As an atheist I would start at the very beginning and ask him to formulate a coherent definition of God based on reason alone. These guys usually take their definition from religious scriptures and try to formulate from their. But remember, scripture based argument is based on the assumption that God exists. We are not obliged to take their scripture based arguments because they are all rival scriptures claiming to be the word of God. Even as an agnostic this method would work. They really need to demonstrate:-
1 - The existence of God
2 - Why a theist and not a deist or pantheist version?
3 - From all the religions why Islam?
When arguments from hadith and verses are presented, you have to remember that they will have explanations already worked out, no matter how absurd they are, simply because it's a given to them, that Mo is a prophet. They really have to cross the first and second hurdle successfully.
Also hadith was compiled more than two centuries after the death of Mo and there was a whole load of it for them to sift through. They obviously had to try to make sense of all the conflicting narrations that came through to them. Hence why there is all the interpretations and trying to fit hadith into certain scenarios that the prophet faced. It still doesn't alter the fact that those statements are attributed to the prophet and that they are spoken words. It's fairly obvious that those things which would on the face of it look crude, would need to be justified somehow. Their attempts are merely trying to square the circle.
Anyhow after my little rant, I must say that you have done well. That admin on the forum is really trying to avoid the issue by throwing rules and regulations at you. The debate is squarely in your hands and the ball is in his court to justify the reasons thoroughly.