Porknpie(Dang. Now I want bacon and some pecan pie!

) Let me answer your second question first, if I may. The Remnant LDS Church is very much different from the “Utah LDS” or Mormon Church. The Remnant LDS came from the Community of Christ, which used to be the Reorganized Church of Jesus Christ of Latter Day Saints. After Joe took some bullets, there was a split in the church. Those that went west, became the LDS church. Those that stayed behind, felt that Joe’s oldest son, Joe III, would be the new leader, and became the RLDS Church(I’m condensing a lot of history here) The RLDS and it’s splinter groups(I’ll just use RLDS as a whole to save time) differ from the Utah group in that they do not believe in polythiesm, and dispute that Joe taught such a doctrine, They also dispute polygmay, teaching that it came from Brother Brigham. With the Remnant LDS, they do have ongoing revelation, and have added to their Doctrine and Covenants. The RLDS also reject baptism for the dead, that God was once a man, that man can become a god, eternal marriage, temple ordinances and the like.
Now, to answer your first question: The “Great Apostasy” is a key tenant to all of Joseph Smith’s churches (and there’s quite a few). Without such a teaching, No Restored Gospel church has a leg to stand on from an authority standpoint. From my understandings of said apostasy, The ancient church (i.e. The Catholic Church) fell away by adding what is viewed as unnessacary things like infant baptism, puratory, the selling of indulgances, the inquisition, and the sort. I left Mormonism not for any theological reason, but rather, I left because while the Mormon Church seems nice, Mormons themselves can be quite “Un-Christlike” (to steal a phrase from them.) I joined the RLDS Church as an attempt to compromise with my now ex-wife (who still is a TBM)