"So contrary to what you think you are doing (i.e. hitting using horizontal hinging), it may well be that you are loading as a hitter (Angled Hinge at the Top) and coming down as a Swinger."
-I think you nailed it there. It's easy for me to take it back as a hitter but coming down is a different story. Any drills for the downswing to ingrain the feel of an angled hinge?
Go to both arms straight and look at the left hand. Make sure it's perpendicular to the angled plane. A visual check is the clubface 45 degrees across the line. I would sit in that position for about 10 seconds. Then, return to address and hit a ball. Keep monitoring the left hand to see if it's cooperating. A little Basic Motion is good to practice.