JOEL - Here are the scriptures:

"Having ascended into heaven, having the bowels of mercy; being filled with compassion towards the children of men; standing betwixt them and justice; having broken the bands of death, taken upon himself their iniquity and their transgressions, having redeemed them, and satisfied the demands of justice." (Mosiah 15:9)

"Therefore ought ye not to tremble? For salvation cometh to none such; for the Lord hath redeemed none such; yea, neither can the Lord redeem such; for he cannot deny himself; for he cannot deny justice when it has its claim." (Mosiah 15:27)

You need to read these scriptures in context with the others around them.
Verse 9 is talking about those who listen and hearken to the prophets words(See verse 11). Christ will atone for their sins, and His suffering in the Garden of Gethsemane and on the cross will satisfy the demands of justice.
Verse 27 is talking about those who rebel against God and do not keep the commandments, and die in their sins, or in other words do not repent of their sins(See verse 26). They will not receive salvation in the Kingdom of God. They will need to suffer for their own sins to satisfy justice.

JOHN - Nephi said that the term "Nephi" would be given to kings of the Nephites from his reign onwards. How come King Mosiah (both of them) and Benjamin were never called by this title?

JOEL - Actually Nephi really did not want to be a king but agreed to it anyway:

"And it came to pass that they would that I should be their king. But I, Nephi, was desirous that they should have no king; nevertheless, I did for them according to that which was in my power." (2 Nephi 5:18)

It was Jacob who said:

"Wherefore, the people were desirous to retain in remembrance his name. And whoso should reign in his stead were called by the people, second Nephi, third Nephi, and so forth, according to the reigns of the kings; and thus they were called by the people, let them be of whatever name they would." (Jacob 1: 11)

Nephi is what they called the kings who reigned in the Land of Nephi from then on. But the Nephites in this land became wicked and God warned Mosiah(the first one)that he should flee out of the land of Nephi and take as many righteous people that he could(See Omni 1:12).

Mosiah was then made king over this new group of people in the Land of Zarahemla. The Book of Mormon doesn't say anything more about the succeding king Nephis in the Land of Nephi. The record only follows the people of Mosiah in Zarahemla. So, it's a different group of people with their own kings.

JOHN - How come Joseph Smith condemned the witnesses and in the Journal of Discourses, Brigham said they were left to doubt their experience?
When the 3 witnesses saw the angel and plates, the plates were in the Whitmer household. How could they (the 3 witnesses) have seen them?

JOEL - I do not trust the accuracy of many things recorded in the Journal of Discourses. It was written by people who recorded speeches in shorthand and then sometime much later tried to reproduce the text. What they wrote was often never proofread by the speaker for correctness.

What the witnesses experienced was made possible through faith and the power of God through a vision. Brigham Young was explaining that when someone apostasizes and loses the spirit of God, it is easy to begin to believe that it was only their immagination and not a true vision.
As David Whitmer said. "In regards to my testimony to the visitation of the angel, who declared to us Three Witnesses that the Book of Mormon is true, I have this to say: Of course we were in the Spirit when we had the view, for no man can behold the face of an angel, except in a spiritual view, but we were in the body also, and everything was as natural to us, as it is at any time" (Anthony Metcalf, Ten Years Before the Mast [Malad City, Indiana, n. p., 1888], 73-74).

I don't see anything contrary about the angel retrieving the plates from whereever they were and showing them to the witnesses.