From what you say it sounds as if the shaft of the control knob has broken. Have a look at these two YouTube videos.
I had exactly the same problem and repaired it successfully.
As others have said it is a common problem but one which Mercedes don't care about and will charge you for sorting it out.
This is a very easy fix and is described in the vid below.
Service books can be faked. If you have the invoices for each service it is better than having a service book when selling.
I took my W204 to an Indie after the MB warranty ran out. £1100 for a service is extortionate.
Get yourself a good Indie near to you and take out breakdown cover...
Yes, mine went on my W204. Easy job to repair after watching the YouTube videos but don't use superglue use araldite. I strengthened mine with wire from a paper clip and is still ok.
I'd be interested if you could post up a link to the 3D printed shafts from Germany.
I had exactly the same problem with my knob !!
I found these videos on YouTube and after warching them took the part out. I was then able to dismantle it and repair it as per the second video and worked a treat.
If, as you say, you could repair a watch this will be easy. Good luck.