What you're suggesting sounds like one dead cylinder due to either ring or piston failure rather than gradual wear of rings and bore.

I would imagine, with all that oil being burned, that it misses. Find out which cylinder it's missing on and do a compression test.

You might even have a burnt piston (which would be better than replacing rings on 6 pistons, having to remove 1 is much less of a hassle). I don't know if it even happens on MB engines or not. Is it possible that something really hard (cracked valve, stone?) got into one of the cylinders and marred the bore to the point where oil flows freely into it and out the exhaust?