I just had the same dilemna you're experiencing a couple of hours ago. I called a local MB mech who recommended that I unscrew the tumbler cover by hand. The cover wouldn't bulge. Its difficult obtaining leverage and a good grip thru a 3 inch opening with your hands. I removed the lower panel to gain access to the cover and used a vise grips to remove the cover turning counterclock wise. It worked. A 15-20 minutes job at most. I also made a U shape tool and tested it on the replacement tumbler to see if it depressed the tumbler's retaining spring enough prior to removing the cover.
Note : You could probably use a tool to remove the cover thru the opening without removing the lower panel. I couldn't