The term "Passover offering" that appears in II Chronicles 35 was frequently not in reference to the Passover lamb, but was in reference to the small cattle (baqar), meaning the oxen or the bullock offered through the entire seven days of the Feast. You have to be very careful as you are reading through this account in II Chronicles 35, because sometimes it really was the Passover lamb. At other times it was merely what they called "a Passover offering." However, they are calling the whole period of Unleavened Bread Passover, just as it is called that in the New Testament, and also in Deuteronomy 16. The proof of that is cattle were offered.
The cattle was not a Passover sacrifice, but called a "Passover offering" because they were being offered during the Days of Unleavened Bread.
In II Chronicles 35 we clearly have two different types of sacrifices: the Passover lambs, and the additional offerings which were boiled or burnt on the fire. They were not part of the Passover as commanded by God, but they were offerings that were commanded by God to be given at other times, including the Feast of Unleavened Bread.
