Do you have a neighbor who has an air compressor and a 1/2" impact wrench? This would be the best way to get the nut loose.
The failure you experienced sounds bizarre. I hope Benzmac or MBDoc will chime in on this one and tell if they have seen a failure like this before. It's sounds so bizarre to me, that I'm not sure I believe the tow truck driver saying that he say the same thing a few weeks ago. Of course, I only see my own cars, the guys in the MB shops have seen most everything, we may find that this happens all the time, if so, I'll be surprised, as well as out examining my pulleys for cracks.
Keep on working through your problem and don't worry about being a beginner. The people on this website will help however they can. Don't get discouraged about your 240D either, you're just hitting a string of bad luck. Get past this, keep your car in shape and it'll run for a long time.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in