Ron,
You have 200 lbs. of R12? MY NEW BEST FRIEND!
You are correct on the need for a strong back and crowsfoot flare wrenches to remove the expansion valve. Some have done it with "stubbies", but not me. Emphasis on the flexible back
Since you are a pro tech on large supermarket systems, you'll know more about this than I do, but I happen to have an expansion valve in my hand as I type this and have disassembled it numerous times, so I'll tell you what I've found (FWIW) : the brass plug is adjustable to open or reduce flow in the TXV valve (nothing different from the old Chrysler H block style) although Mercedes gives NO info to users or techs on this fact. It uses a ball and spring on an orifice seat, spring tension adjusted by screwing in (or out) the brass plug. Just stick a nail set or small drift in the eccentic hole and twirl. The plug DOES have an "O" ring (looks like a standard 1/2", but probably a 12mm.) You'll undoubtably have it's equivalent in your parts bin. Mark the plug and block before unscrewing it, so you get the correct tension when replacing, and don't lose the little ball! Good luck