The 220D is shut off mechanically via the same push/pull knob mechanism used to glow the plugs and start the engine. When you push the knob all the way in, a cable should consequently push on a lever at the injection pump to shut off fuel to the injectors. Sometimes these cables can become mis-aligned, or a piece at the lever end can fall off, restricting the ability to shut off fuel. I would inspect this cable mechanism.

These cars have a rather simple vacuum system (relative to the later models), so I'm not sure what hoses are disconnected, or whether they would impact any critical functions other than the vacuum pods for the climate control system under the dash--again, very simple.

I don't remember where the diff drain is, but I'm sure there is one.

I'll bet the headrests are interchangeable with later 115's and early 123's.