I'm pretty sure they're out to make money on your car.
I've seen numorous bullitens instructing that ignition coils are not to be replaced unless they are deamed to be faulty by means of osciliscope patterns or physical damage (broken electriacl connectors, corrosion, etc.). Another point is that they used a fuel additive. MB specifically state that additives of any kind are not required.
So they've replaced the injectors, replaced ignition coils, replaced spark plugs, updated the engine ecu software, cleaned the air intake, drained the fuel tank and given you a fuel additive
............How confident do you feel about the diagnosis skills of the people working on you vehicle when you are out of warrenty and you are paying the bill
So they didn't actually pinpoint an exact fault, sounds like they just replaced everything.
Verbal recommendations should never be passed on to customers unless they can be backed up in writing, it doesn't matter what industry you are in.
As long as your fault is fixed, all is well. Just be mindful when you are paying the bill next time.....ask lots of questions!