Just to finish off this thread I bought one of these:
It works for charging with no errors on the iPhone, but does not add any additional music connectivity.
I also tried to find cables for my 2015 VW EOS, which has a similar plug. I bought 2 or 3 and none of them gave me any new infotainment system access, and only one seems to actually charge.
So you guys are exactly right, just use the USB ports for charging and Bluetooth for audio. The B-class already has a USB port next to the funky media connector, so nothing is gained by a cable. In fact for me, plugging a USB adapter plug into the cigarette lighter makes even more sense because the phone lives out where I can reach it most of the time.