GotAGoodPoint,
“How do we know that זה refers to matza & marror, and means by night, maybe it refers to the קרבן פסח and זה applies already by day.”
I’m not sure I understand your question. The korban pesach is slaughtered during the day on the 14th of Nissan, but it is eaten by night with matzos and maror. So wouldn’t the time that זה refers to be night by the korban pesach as much as with matzos and maror?