Although in general, the Gemara in Pesachim 28a and 29a tells us that Chametz that a Jew owned over Passover is forbidden for benefit, there may be some exception.
29a tells us that the nature of the ban is due to a "Rabbinical penalty" imposed.
The Meiri quotes an opinion in the name of the "Chachmei Luniel" that chametz owned by a Jew over passover is permitted as long as the Jew uttered a sincere nullification before Passover (bitul). The Meiri disagrees.
The Rambam (Chametz U'Matzah 1:4) (agreeing with the Meiri) says that it is forbidden even if the ownership of the Chametz on Passover was a total accident or onus and even if the Jew was not in violation of owning or seeing his Chametz (because he made a valid bitul). This is because the Rabbis issued their ban so that people should be more careful.
Shulchan Aruch echoes this by saying that a Jew's Chametz found over Passover is forbidden for benefit even if it was an accident or onus. (O.C. 449:3 and 5)
However, the Be'er Heytiv on 449:3 brings differing opinions. He explains that the language of the S.A. leaves some room. Does a person who performed a valid bitul and afterwards found Chametz by accident, need to treat the found Chametz as forbidden or permitted? (The lenient opinions would claim the S.A.'s strict stance against accidents only applies if they accidentally forgot to make a valid bitul as well.)
He says the authorities are split on the question. He brings the opinion that in the case of a great loss, you may rely on the lenient view to permit benefit (sale). (but perhaps not eating it outright?)
The Mishnah Berurah 448:9 brings a case that happened. A Jew left wheat in a mill by a certain Gentile (the wheat was not Chametz yet and he left it through Passover). The Gentile wanted to do the Jew a favor. So, he ground the wheat for the Jew on the 7th day of Passover and made it into Chametz bread for him. Right after the holiday, he gave the Jew "fresh baked" bread. Since it was the Jew's wheat and it became Chametz on Passover, can the Jew benefit from the bread or eat it now after the holiday?
One opinion (a posek named "Beis Hillel) is that it may be sold but shouldn't be eaten.
But, the Beis Meir holds that although accidental ownership of Chametz on Passover is forbidden, not all accidents are the same! Since the event of the Gentile deciding to make it into Chametz on Passover and deliver it to the Jew is so far fetched, no one could be required to think of this happening. Therefore, the Rabbis never penalized cases like this one. So the Jew may eat the bread without reservation. (This is true even though there was no bitul!)
So too, the OP's case is so strange and unforeseeable, that no one should be penalized to "be more careful" to find out that every Gentile is really not a long lost Jew! Furthermore it is a case of extremely great loss as 200 households would suffer financial loss and inconvenience! (they all also presumably made a valid bitul before the holiday.) It is also a famous general rule throughout Gemara and poskim that "Milsa d'lo shechichah, lo gazru bei Rabbanan". The Rabbis did not enact decrees upon far-fetched, unlikely cases.
Therefore, although Chametz is usually forbidden for benefit even due to accident or onus; the case of the OP would seem to be permitted in that the 200 households may at least sell the Chametz or even eat it themselves due to all of the above exceptions.