I don't know if this is the problem for you, but when I had mine apart to get the tumbler repaired, upon reassembly, I didn't get it aligned correctly and it missed the little loop on the latch mechanism that the handle pulls to release it.
On mine, I had to loosen the handle again, slide it forward and then back, hooking it in the loop.

In your case, is there any chance that it was installed with the handle barely catching the latch, and it's flexed the metal enough over the years to have popped out?

Another thought, maybe just pulling the handle, getting into the lock mechanism with some cleaner and lube, and working the mechanism a few times might check the function, and possibly free up some gunk jamming up the mechanism.

From looking at how my car is assembled, it appears that you could swap door handles from right to left, thereby getting a functional door tumbler from the passenger side, and using it on the driver's side.

You might also consider checking if any of the locksmiths in your area will work on MB lock tumblers. Neither of my door lock tumblers worked, so I pulled one, took it and the key to a good local smith who rebuilt the whole core of the lock cylinder and keyed it to my key.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...