I didn't read the whole thread but either turning the engine off at WOT, or opening the throttle to wide open as soon as you turn the engine off, should cure the backfire problem.
Some people worry about turning the engine off at WOT (for fear of hurting it somehow) but if you think about it, the engine doesn't do anything any differently than it does while running at WOT....other than not having a spark to ignite the gas/air mix.
The purpose of the WOT throttle position at shut down is for the momentum of the engine to force the remaining air/fuel out of the muffler so it can't ignite (which causes the backfire).
Shut 'er down Buford....she's slangin' oil !!!!