Give some penetrating oil a shot and then the big breaker bar again. Gotta snap that bar and not torque it, if you get my drift. Two man operation here. One holds the torx fitting in the head bolt, and the big guy snaps that breaker bar with a quick motion. Either the bolt comes out or the bolt head snaps off. Good luck.
Dude, just call your local M-B dealer and ask to speak to the service department. Or try an M-B specialist....you know, the kind with the white lab coat, surgically clean workshop, and answers to Hans or Harmutt. I'm sure these guys do it everyday and will tell you some simple thing that 'll make you go "Duh-Oh!". Good luck.
One thing that has worked for me is to soak a piece of cloth with penetrating oil, place the cloth on the bolt(s), and walk away for a day. You may have be a bit creative with the cloth, but the idea is to keep the bolts "wet" and give the oil time to work in as deep as possible. Then use the long breaker bar to snap the head off the bolt as you cuss like a sailor.
I'm sorry. I really didn't mean to be insulting. You are one of my favorite motojournalists, and I thought that it's likely the best course of action. Kind of like your opinions on various bikes. Some of them just aren't worth sinking resources into. I hope I'm wrong and it works out.