these are probably dumb ideas but who knows...get a small hand held electric sander, the kind that vibrates, hold that behind the key so that it causes the key to vibrate in the tumbler, gently work that key back and forth, use lots of lube on the key. Also if you have other copies of the key, try those. If you have access to compressed air, hold the nozzel over the key hole and blow the air in there hard, maybe that will loosen or free whatever it is thats messed up. If everthing fails, take a long very thin screwdriver, insert it in the keyhole and bend the screwdrier handle back and forth around and around to bend the tumbler and move it around a bit,then do the key. after awhile you might as well try to use greater force since its going to be drilled anyway...heres a messy one..take a powerwasher, hold the nozzel over the key hole and spray the high pressure water in there, then try the key.
I have the same problem right now, it was $455 total cost to replace tumbler and lock and get a new coded key. You may want to consider getting a new tumbler with just a generic key then settle on using two keys for the life of the car.
Good luck

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1996 E320
2000 C230 Kompressor
1988 190e 2.3 - 225K miles, owned for 7 years. I cannot say enough good things about this car. Very well built, even at 225 it ran like new.