How is it that the Lord may make this covenant that through baptism sins will be forgiven?
He may make that covenant because he will be the one who creates the conditions for the fulfilling of the covenant. Once again, the Book of Mormon is dealing with Jehovah/Christ as their God explicitly. The Lord who teaches them and communicates with them is the very one who will become incarnate and sacrifice himself for mankind. He may make this promise because he will fulfill the requirements of this promise.
“A Place at My Right Hand”
The blessings promised are not only for this life, but will transcend this life. In verse 23 the Lord notes that some blessings will be granted "in the end." The "end" is the time when the purposes of the earth are fulfilled. It will be the time of the triumphal Messiah.
Translation: The imagery of the right hand comes form the ancient conceptions of the right hand as good and the left hand as evil. The Latin word for left is siniestra which becomes the English sinister. Thus it is a blessing to be on the right hand of the Lord. This conceptual division between the left and the right is well known from the Old World, but less so from the New World.
There is little literature on the meaning of the left hand in Mesoamerica, but there are hints that not only was it not considered evil, but that it may have been a sign of a connection to the powers of the other world. On a very simple plane, the left-handed warriors of the Mexica were considered the most fearful. One explanation has always been that they fought differently, and in that different posture lay a military advantage. However, it is also possible that there is more to it than that. The name of the Mexica tribal deity was Huitzilopochtli "hummingbird on the left/of the left." Certainly that context is positive for the Mexica. A fascinating possibility comes from an analysis of the various stelae at the site of Izapa, a much later description of the underworld from the Codices Matritenses, and the Mesoamerican fascination with mirrors.
In Izapa, there is a preponderance of actions effected with the left hand. Either the Izapans were statistically left-handed more than any other known population, or the depiction of actions by the left hand had a different significance. There is a description in the Codices Matritenses of the Mexican description of the underworld. That description focuses on the underworld as a place of reversals, where things are reversed from the real world (Codices Matritenses Fol. 84r/v). As a final point we have the Mesoamerican mirror of polished obsidian or hematite. Mesoamerican mirrors have symbolic connections to the primordial waters and the underworld (Karen Bassie personal communication). The mirror effect is so well known that we understand a "mirror image" to transform right to left, and left to right. In this context, the left hand may be a representation of a connection to the underworld, just as is the image in a mirror.
What does this mean for this passage? It simply means that when Joseph translated the conception, he used imagery that was familiar to him, and not necessarily imagery that might have been at home on the plates. Of course we do not know precisely what the plates would have held, but in the social context of Mesoamerica, it would not have been likely that the "right is good/left is bad" imagery of the Old World would have survived long enough to make it onto the plates at this time period.