The only problem I have with that (spiral flow) theory is that, when you actually put a cotton thread streamer right behind the prop, and watch the tail end of it down by the fin, there ain't enough 'spiral' to tell which side of the fin it will go...The problem is that great big 'airflow splitter' called a wing, which tends to prevent any spiral flow happening. The other objection I have is that you find the exact same thing happening with a pusher wing, where the only thing behind the prop is...nothing. Torque, of course, is always there, so long as the prop is turning.
Evan, WB #12.