Hi guys,
Just so everyone knows - I just realized I got credited for the miles corresponding to these old flights on Air India and Air China that happened up to 9 months before I even enrolled in Miles and More.
In the end what I did was: write a nice letter to Lufthansa explaining that I had lost all boarding passes but with a receipt of the flight tickets and one baggage slip that happened to still be hanging on my luggage. They sent a very courteous letter saying that they were going to check with their partners, and a couple of weeks later I had been credited with 18,000 miles.

Thanks for giving me good advice at the time :-))