1. Spark plugs: I checked the gaps in all plugs and ran into this problem: Plug No.2 was almost impossible to get out. I used a braker bar, penetrating oil and hot engine (cold did not work at all), and this was the only plug which only came out with hard work until the final exit; after gapping, it did not want to go back in, either, so I only screwed it in about half way, and it was tough all the way. Now that I bought new plugs, what should I do? If this was the symptom of a cross-threading, I rather screw the new plug also only half-way in and not have the expense with heli-coils, etc. Is there anything major wrong with this approach? The car is not driven very much. By the way, the threading of the difficult plug did not show any signs of damage/stripping or dirt, so I assume whatever damage there is is in the block, and a new plug will also stay in tight.

2. Anti-freeze: A couple of years ago, an independent shop specializing in Mercedes replaced the radiator with a Japanese, generic one and used Peak-anti-freeze (the green stuff) to fill it back up. When I asked about Mercedes' anti-freeze, they just made fun of it, saying that it is a scheme to make more money, that the green stuff also has all the protection additives for all the metals used under the hood, etc. What is the final wisdom about this? What chemicals would Mercedes use that nobody else does? I am coming up to a renewal of the coolant, what should I do and why? Somewhere I saw a reference to a cheaper alternative, Zerex-brand from NAPA and Autozone and the containers don't say anything really different from Peak or Prestone, anything else?

3. Brake sensor: One front brake sensor wire is broken off too close to the caliper to fix the wire. I assume that this does not affect the ABS, so I should be ok if I check the pads every couple of years. Anything wrong with my thinking? How could I reattach the wiring?

4. Distributor: My shop manual shows a vacuum advance attached to the distributor, but there is nothing on the car, and nothing seems to be missing. Do I have the wrong distributor?

5. Timing: The timing seems to fluctuate a little; it was a lot worse before I found a broken vacuum-thermo switch (50 degrees) and replaced it. Is there another place I should look for something wrong?

I appreciate any good advice.

Plugs: don't force them, aluminum heads and all. loosen and spray. sometimes there is a partial thread which protrudes into the CC and when you back them out it drags carbon; anyway you don't want to damage the threads. Mine were hard to get out but I stuck with it and got them out without damage. Just keep loosening, moving back and forth, and spraying.
Anti-freeze: I use the MB dealer's stuff; it isn't too high. I understand it is Zerex G-5. Anything good for all-aluminum engines should be OK. Use good water. My car had the green stuff when I got it, no telling how long it had been in there. All OK here 5 years later.
Brake sensor: you don't have to have it, but it would be nice. there might be a short replacement harness, etc, but I have not had this problem yet.
Dist advance: As I recall the timing is controlled by computer and all the distributor does is distribute. Install it on TDC #1 and 'puter does the rest.
Timing: make sure the crank damper isn't loose (probably isn't) but I don't know how you are determining it is fluctuating: timing light?
Check your cam drive chains for wear: max 7 degrees as I recall. the plastic guide shoe on the driver's side can break and get between the chain and sprocket, causing a big engine wreck you don't want to have.
Hope this helps you until someone more experienced replies.

Oh, Oh! I almost forgot: for the timing issue,check out the crank position sensor. It plugs into the ignition module on the driver's side frnder well, has a green wire, and the other end is underneath the exhaust crossover at the rear of the engine. You access this end from underneath the driver's side, behind the wheel. Just reach up and you can feel it on the top left of the bell housing. It has a plastic, snap-on cover, and is held on with a small bolt. it may be getting goofy after years. If it has never been changed it may be good to be proactive, as it is not too expensive compared to a tow truck, ask me how I know.
I don't know that it could affect timing, just might though.