I am not aware of a single fuse for the seat belt extension arms. The best thing to do is to remove the rear seat (bottom and back support). In the top corner of the side panel you should see a small computer box with a 8 or 10pin connector. Unplug it this will disable the ext arm system. You have to do it to both sides. That's the best thing you can do for now and you can put back fuse to power whatever else is fused on it. As far as repairing the system, it gets a little complicated. The problem is very simple two small worn out plastic gears. The big problem is that you can't order it. Mercedes sells it as an assembly (don't ask me why). I had this problem with my car. I searched all over the place for the gears and ended up finding very similar ones in a Panasonic printer. Did a little modifying and it works like a charm for two years now.
The problem with your headrest is inside the seat. You headrest release mechanism broke. Usually breaks when someone tries to pull out the headrest without pressing the hidden button on the rear seat panel (check owners manual). Anyway, you can buy the part from Mercedes and replace it by removing the rear panel of the seat.
For your transmission problem I would check the Bowden cable. It's located under the air filter housing. The proper setting is no tension in idle position. I would also recommend a filter and fluid change.
Your mirror might be fused on the fuse that you pulled out. I'm not too sure.
Yes, most of the 300E post are applicable to your 300CE. 88 was the first year and they shared the same engine (103).