Mormon Dilemma 920 Mosiah Teaches no Baptisms for Dead

Mosiah 3:25; “And if they be evil they are consigned to an awful view of their own guilt and abominations, which doth cause them to shrink from the presence of the Lord into a state of misery and endless torment, from whence they can no more return; therefore they have drunk damnation to their own souls.”

2 Corinthians 6:2; “(For he saith, I have heard thee in a time accepted, and in the day of salvation have I succoured thee: behold, now is the accepted time; behold, now is the day of salvation.)”

It makes for a hard day at the office for all Mormons when they have to explain the verse in Mosiah. 

Why would ‘the most correct book’ contradict what they do in the temples? How can the Book of Mormon be from God if it does this and which teaching is correct?