As the bulletin states, the indexing is achieved by using the correct plugs and applying the correct torque of 23nM. I've done them on my 2014 ML350 twice (60K interval) and now have 135K on the car with no problems. As for access, the WIS says to remove the upper section of the intake manifold. On the ML that is really difficult to do due to the shape of the firewall (there is a removable section that is a royal PITA to remove). The sedans are much easier. I do not remove the upper intake. It is tight reaching/releasing the coil wire connectors under the intake but doable. Drivers side is the more difficult. The passenger side is easy after removing the air filter housing. Buy the plugs from the dealer. Here is a pdf that I found helpful - for SLK but it worked for me.

https://www.slkworld.com/threads/changing-spark-plugs-or-ignition-coils-on-the-r172-3-5l-v6-m276-engine-version-2.578791/