I just went outside and tried my 88 300E trunk lock. This is the way it works:
With the slot vertical the trunk is synchronized with the central locking system
With the slot horizontal, the key only comes out if you turn it all the way clockwise (to the right), and the trunk lock is no longer synchronized with the central locking system. Turning the key to this position also locks the doors and gas filler cap.
Turning the key counter clockwise unlocks the car doors, gas filler cap and the trunk.
Turning the key is pretty hard and sluggish. I was a little alarmed and will lube the lock next. But the resistance is to the mechanism involved in the central locking system on my car, not the lock tumblers. I can feel the difference. There is a persistent resistance to turning the key all the way through the stroke, except at the very end of the full clockwise motion to lock things. Once the locks all clunk, the key turns, even if the range of motion left is very small, much easier.
If I was you I would get some good spray lubricant like WD-40 and flood the thing, a couple times, and then force it with the key. If you break the key off, well, that's another $20, but I think the key is pretty stout and should be able to force the lock to turn. Hope this helps, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)