Have you considered that it might be the ignition switch? I take that mercedes took the new battery back when it didn't clear your problem?
and refunded you for the key?
The other thing worth considering is the starter lockout relay :-
http://www.aclassinfo.co.uk/mypage.66a.htm Bert Rowe's-mercedes-Benz 'A'-class info. Replacing Ignition contact switch.
