You ask about the RLDS understanding of Christ, of salvation, what their major error is, and how to lead them to faith in Christ. I address much of this in the final 2 chapters of my book, Part Way to Utah: the Forgotten Mormons, chapters 17 & 18 "Understanding the RLDS Dilemma," and "Helping RLDS to Victory." You might want to check out these 2 chapters for some ideas. Their major error is that they have accepted the leadership of a false prophet, Joseph Smith, and thereby have accepted his false scriptures and doctrines as well. Instead of correctly discerning that Joseph Smith was in error, they hold his doctrines up as God's ultimate truth. And because they have accepted Joseph Smith, they have also accepted the counterfeit Jesus who is portrayed in Joseph Smith's own scriptures, who is not the Jesus of the Bible. They therefore have a distorted view of Jesus, and of salvation. They mistake Joseph Smith's scriptures and doctrines for the gospel. And since they have responded to Joseph Smith's teachings, they therefore feel they have responded to the gospel and to Jesus Christ, and therefore have salvation.
In chapter 17 of my book I speak of spiritual immunization. In his commentary on the book of Hebrews, Biblical scholar F.F. Bruce says,
"People are frequently immunized against a disease by being inoculated with a mild form of it, or with a related but milder disease. And in the spiritual realm experience suggests that it is possible to be 'immunized' against Christianity by being inoculated with something which, for the time being, looks so like the real thing that it is generally mistaken for it."
This is precisely the dilemma facing RLDS people. Joseph Smith borrowed just enough material from the Christian church, for his own church to be mistaken as Christian. And since RLDS people feel they already have God's ultimate truth, why should they listen to you? It's you who should be listening to their superior revelation. People come out of the RLDS church and come to Christ generally in one of two ways. (1) If they are not overly steeped in Joseph Smith's teachings, they may sense a different "Spirit" among genuine Christians than they do in the RLDS church. In response to God's Spirit, they may leave the RLDS church behind and join with genuine Christians. While this does happen, it doesn't happen all that often. (2) Most RLDS leave their church and seek out genuine Christianity only after becoming disillusioned in Joseph Smith, his scriptures, church, etc. This is often a painful process, and many RLDS people simply don't have the personal courage to go through with it. It takes an unswerving commitment to the truth--no matter the cost-and a deep faith that God will get you through it. When confronted with their own personal moment of truth, many RLDS simply put their head back in the sand. I've had some tell me as much.
If you want to lead an RLDS person to faith in Christ alone, you may first have to lead them away from Joseph Smith and his teachings. So you should become familiar with their beliefs and be able to demonstrate precisely why they are un-Biblical. But above all, you'll need to pray for them. Perhaps for a long time. The battle for the souls of RLDS people is primarily a spiritual battle.
"For our struggle is not against flesh and blood, but against rulers, against authorities, against powers of this dark world and against the spiritual forces of evil in the heavenly realms." (Eph. 6:12)
Paul Trask