Dear Ubiquitin,
This is a very late response to what I was thinking when I posted. It was from my memory then and it still is. I don’t know when I will learn the topic again. It deserves proper sources.
I understood the scenario as, the Jewish Distributor that refused to sell his chometz has sold to a variety of stores that we can’t clarify exactly. Assuming that we have a mixture and are attempting to use rov, it would not permit the distributor to do so, because he could have sold at the point of issur. Once there is a matir at any point, we don’t use rov where the matir is a possibility. When one willfully ignores the matir, then we do not allow them to use the rov at all.
There can be a strong argument that we do not care about the mixture to begin with because the consumer isn’t doing an issur or knowing that the purchase is assur. But if we assume that the retailor’s stock is all mixed to the point that there is nothing stopping us from going after rov, than the phrohibition from the distributor should never be removed by making a mixture out of it. Because that phrohibition was avoidalble with a clear matir.
PS I’m assuming that the sale of an assimilated Jew’s chometz is valid.