Well I'll be a monkey's uncle! I finally got around to trying again. Took it apart, but still struggled to remove the earpiece. Eventually I prized it out with a very sharp knife and... it split apart! One block with the magnet and connectors came off but the membrane with the wire coil stayed stuck inside. The wire is thinner than hair (though it may be just my impression) and the magnet block was precariously hanging, literally by a thread. "That's it", I thought, "I will have to beg asperguy's forgiveness for publicly pointing out his 'senior moment' and try a last ditch effort with his spares."

With a small heart (an idiom from my first language meaning "with trepidation"), I put the whole thing back together and... IT WORKS! I fixed it! The sound is as clear as can be. I am a magician!

But that's it. Some of the screw became so loose after repeated disassembly that I may not be able to put it back together again. So if it breaks again I will probably fix it with a big hammer.