"and the first-born": and not one whose first-born status is in doubt.
"the hated one's": i.e., even though the first-born son be the hated one's. Variantly: Scripture apprises you that the first-born son shall be the hated one's.
(Devarim 21:16) "Then it shall be on the day that he causes his sons to inherit": Inheritances are adjudicated in the daytime and not at night.