The Beis Yosef includes Muslims in the exception. The Tur and Mechaber use the term Yada Betiv AZ. That would answer my question about Itay. Since he just decided to drop it all he is still a Yodea Betivan.
I don’t know the percentage of athiests in this field, but I assume that an athiest is Lo Kol Shechein not a Yodea Betivan.
Selling it does not help, unless it is being sold for scrap metal. In this case it is being sold as something special. The seller knows it is being cared for. However, being that we probably aren’t talking about the idol itself, only its utensils, even disregarding it orally helps, without defacinng it.
There is a concept of Bitul by abandonment. Naftush is applying this Bitul when he says that AZ was dropped. The simple Halacha is about a specific idol, though. Also, it depends how idol worship was dropped. If it was one day to the next then that can be considered abandoned. But if it was a gradual process then it is not necessarily so. In order to apply this Bitul you’d have to say that there was sort of a class action dropping of idols. And then you will have to make the stretch to buried items Delo Chazitei Udelo Chamitei.