I think the OP has a point about the manufacturer not being able to supply certain spare parts, this is obviously a potential issue e.g. if your Distronic-equipped car is involved in a crash and needs new radar sensor etc.
Said that... we are talking about one order that the manufacturer was unable to supply... this can happen to any manufacturer. It does not imply that there's an endemic issue with Mercedes Benz spare parts.
Regarding the attitude of the sales people at the dealership.... sadly there are too many complaints of these nature, so I think we should accept that there's an issue with quite a few dealerships on that front.