My closest experience is on a w115 220d, so take my advice with a grain of salt:
1) Use a volt meter while a friend activate the GPs. On my 220d, the voltage arrives at the rear of the engine and ends at the block in front of GP1. So starting at the back, check voltages along the way. Normally, it is about 1V drop across each GP, and 2 V drop across the wiggly connector bars/wires. This allowed me to determine the first GP that had problems. I ended up changing them all out, so this info was academic in the end.
2) On those two middle plugs, the IP is in the way enough that you cannot easily use sockets. So I bought a regular box wrench for those. It was slightly slower and meant I could not torque them going back in, but was no big deal.
3) The loops kind of do a little reaming as you pull them out. The reamer for the old loop plugs is not cheap. I did my best with a screwdriver.
You can read about some of my experiences here:
Glow Plug Replacement: a beginner's perspective
and
Beru loop style glow plug source?
In post 11 of that second thread, I give the dimensions of the official MB reamer for the 220d OM615.