Let me try to give you some insight on a couple of your questions.

#1. It's called "blow-by". It is actually a mist of very fine particles of unburnt fuel, oil droplets and possibly bits of exhaust gasses. It can be caused by, worn compression rings, worn pistons or cylinder walls, leaking valve guides or a bad cylinder head gasket. Some amount of "blow-by" is normal. It is usually captured by the tube leading to the intake manifold or air cleaner. Since this is a diesel, there usually is only sufficient vacuum present to pull the mist into the intake process above idle.

#2 A "glowplug" is basically like the element in a toaster oven. In a Mercedes, it's easiest to think of it as the filament in a light bulb. When electricity is applied to the light bulb the element glows red/white hot. This "preheats" the chamber and inflowing fuel/air mix. Diesels "fire" on compression. Compression creates heat. The glow plug simply warms up the elemnts involved to expedite the process. Once the engine starts, the cylinder walls and head heat up and the glow plug is no longer needed to ease the process.