I thought the real Mercedes parts had a one year warranty?
Just speaking in general from what happened to me. I had the symptoms of a stuck key lock tumbler. But it turned out that the inside of my steering column lock was broken and in more then one place and one of the contact points on the actual ignition switch was burned and it was the arm itself that was making contact.
Replacing the steering column lock with a new one cured the issue. But before I installed the Steering Colum Lock I deactivated the lock spring plunger so that the steering Colum never locks. The advantage of that is you don't turn the key against that spring tension.
About a year and 1/2 later the Lock Tumbler broke. I bought a cheap Meyle one which has been working fine and I also saw a new real Mercedes tumbler and key on eBay and I bought that but never used it.
In your case you could try deactivating the steering Colum lock main spring.
If you do it the way I did you will need to remove the Steering Colum Lock.
After that you turn the Tumbler so that the steering column locking plunger retracts inside and you press in that (brass colored?) button on the side and turn the Tumbler back and let the button get locked in the depressed position.
Make sure the button is all the way and desegregate the area with brake cleaner and let that dry. Fill in the button area with the slow curing JB Weld epoxy and tape over that so that the epoxy will follow cylindrical contour.
Let it cure in a warm area for about 4 hours. Remove the tape and fill in any voids in the epoxy with more epoxy and re-tape. Remove the tape and do any contouring needed on the epoxy so the column lock will return back into place. After that you won't be turning the key against that spring tension but of course won't be able to lock your steering column.
There is an attached picture in post #13 and a red arrow points to the pin that needs to be depressed and epoxied down as if someone one wants to do as I mentioned.
Removal of steering lock bar in 83 300sd