I checked with a shop-owner friend of mine, and he said:

“We have done plugs on this engine, and there was no bulkhead removal. However I could understand that would be possible on an M class. There is a reason that we charge either three or four hours of labor to do this job. But I really don’t think you need to take the intake manifold off, I believe once the ECU is off on the driver side and the air filters off on the other side you can get the coils out and get to everything. One key to success is having really good tools and a variety of extension lengths.
it’s not just the ECU but it’s whole bracket comes off. I do remember I had a customer who tried to do his own plugs and had started by trying to take the intake manifold off and I said: “I don’t ever do that...” “