Do a "search".

The Mercedes keys are steel and the wear takes place in the lock cylinder.

No fun to replace... if you can't turn the key...LOTS of information.

New lock cylinder (with keys/keyed to your V.I.N. less than $100.00)

It ain't your switch (ELECTRICAL- behind and mated mechanically to the lock).

The alarm system has no bearing on the BAD luck of no one noticing that the
tumblers were on their last legs.

This tumbler system has to "Throw" a 1 Lb. bolt home into the steering column every time you remove the key.

Symptoms are "Funky" engage/disengage of ignition."Feeling" the difficulty
of turning the key is a very subjective (and learned) experience.You don't
want to become an expert at this sensitivity.

GOT a Mercedes with over 100K "on the clock"? ...Get a new IG lockset.