Re: US Dentist left Cotton under husbands crown... irony!
IMO, placing a piece of cotton between a metal abutment and the inside of a crown, and placing one between a filling and living tooth is a big difference. The cotton between an implant abutment and crown doesn't touch living tissue. It is sealed in between two neutral and inanimate objects. Nothing can decay.
But, cotton on top of a filling? The filling itself would touch enamel or dentin. Shoving cotton in between there would surely cause eventual decay.
I am so sorry this happened to your DH. A written statement as to your current dentist's findings would be prudent.
My best to you,