Quote:
most effective method of removal I have found is with stem fully open/retracted
Exactly what I was thinking. The stem must be retracted and must turn with the body of the diverter as the body is unscrewed. If the stem does not turn with the body as it is unscrewed, you are extending the stem as you unscrew the body. Since the threads between the stem and the body are courser than the thread between the body of the diverter and the valve body, the stem would travel father inward (extending) than the body would travel outward. That would therefore push the stem up against the valve seat with great force.