In a recent previous post, "'From Whence They Can No More Return': What Lehi Teaches Us About the Book of Mormon's Harsh Language on Hell," I noted that a passage in Lehi's farewell speech gives insight into the limits on the "eternal" nature of hell that has confused many readers of the Book of Mormon. Such limits, of course, are consistent with an important revelation given to Joseph Smith Section 19 of the Doctrine and Covenants where we learn that while God's punishment can be called eternal since He is eternal, that does not mean that those who experience eternal punishment are never freed from their pains. 

The Book of Mormon provides another example that should immediately help us calibrate the intent behind some uses of the words "eternal" and "endless" in the scriptures. In Mormon 9:13, Moroni writes, 

And because of the redemption of man, which came by Jesus Christ, they are brought back into the presence of the Lord; yea, this is wherein all men are redeemed, because the death of Christ bringeth to pass the resurrection, which bringeth to pass a redemption from an endless sleep, from which sleep all men shall be awakened by the power of God when the trump shall sound; and they shall come forth, both small and great, and all shall stand before his bar, being redeemed and loosed from this eternal band of death, which death is a temporal death. 

All men must die, and by so doing, we enter into a state that Moroni describes as "endless sleep" where we are bound with the "eternal band of death." It would last forever were it not for the redemption from death made possible by the Resurrection of Christ. Though the state of death, a function created by the Eternal God, can be called an eternal and endless state because He and His works are endless and eternal, it will be temporary for all of us.  If physical death can be called endless and eternal and yet be temporary, entrance into hell, spiritual death, can also be temporary. You may disagree with the way those words are used, but the Book of Mormon itself makes it clear that these words in the context of death and hell may need to be understood in the way they are explained in Section 19 of the Doctrine and Covenants.  

Update, Jan. 11, 2020: Robert Boylan has provided some valuable articles dealing with the meaning of "eternity" in the scriptures that can add further context to some of the issues raised here. Please see his "Resources for 'We Agree with Moroni 8:18' day (18 August)" at https://scripturalmormonism.blogspot.com/2018/08/resources-for-we-agree-with-moroni-818.html. Among these, be sure to see "Moroni 8:18, Psalm 90:2, and the Latter-day Saint Understanding of Deity."