Non-Chametz Products on Pesach - Yechaveh Daat 1:11

Can a person eat butter or cheese on Pesach if it was manufactured before Pesach (and presumably did not have a Kosher l'Pesach hashgacha on it)? This issue hinges primarily on whether or not we say chozer v'neiur, which means that something that was batel before Pesach can "come back" and become assur on Pesach. In other words, if a small amount of chametz got into a food before Pesach, when it would be batel, can it become no longer batel once Pesach starts? There is a debate on this point, and while Sephardim take the lenient road, Ashkenazim follow the Ramo who notes that there is a view to be strict when it comes to things such as cheese, fish, and dry meat (hence my Rumanian hot dogs are being put away). In the second half of the teshuva, Rav Yosef speaks about not being overly strict in halacha. While a person can decide to be strict for himself, once a person is in a position of being moreh halacha for others, he should only be strict if he is doing so based on sources, and not simply on personal preference. He ends by citing the Teshuva MeAhava who says that a person who is overly strict for others by mistake has a harsher punishment than a person who is erroneously lenient, since the former person sinned to his fellow man.