I'd go with a version of your larger affair scenario. My main difference in detail would be that I think Saddam's Oil-for-Food coconspirators were helping arm him with WMD as well as profiting from Oil-for-Food corruption. But apart from that I agree with your gist, and your bottom line that Oil-for-Food-related crimes (and profits) were most likely the heart of the motive.
Has to be the heart of it. And I have no confidence Fitzgerald's investigation is going to do anything postive to shine light on the truth.