I am hoping to find an authoritative guide for troubleshooting and repairing the "key won't turn" problem.

So far I have found these links, if anyone has others, particularly an authoritative guide, please let me know:

http://www.dieselgiant.com/repairignitionlock.htm

Steering lock "repair" (with pictures) - Mercedes-Benz Forum

http://www.benzworld.org/forums/w123-e-ce-d-cd-td/1496216-steering-shaft-lock-removal-help-needed.html

cheers!