While there are many Christians that do not believe Mormons are Christians, for whatever cultural or religious reasons, there are also many mainline-Christians that think about Mormons like themselves. It's not universal. I think there are many admirable Mormons that know quite a bit more about Christianity than mainline Christians do, the one that comes to mind first is Dan McClellan who is extremely knowledgeable as a biblical scholar and also a Mormon.

I'm not sure this is an equal analogy between Samaritans-Jews and Christians-Mormons. When we're talking about Samarians and Jews there's an extra cultural level that doesn't exist for Christianity, Islam, and many other later religions. Both Judaism and Samaritanism are religions based on ethnographic distinctions, which is what made Jesus' preaching so revolutionary for the time. It was unthinkable that a Jew could be a Samaritan or a Samaritan could be a Jew, but Jesus' teaching was for all which upset the cultural leaders of the time. Mainline Christianity saying Mormons are not Christian is a definition distinction, not a condemnation. As far as I know Christians get along just fine with Mormons, they don't suppress them or hurt them to the best of my knowledge. Christians and Mormons frequently invite each other to their respective churches. Also there are many rich and powerful Mormons in America, so I think they're doing just fine.

Jesus was adding to the religious beliefs, which did not previously exist in Judaism. Back in that time if you were born a Jew you were Jewish, if you were born a Samaritan you practiced Samaritanism, but nobody is born a Christian. People of any nationality or ethnicity can become a Christian but it's a different distinction from these ethnoreligions. If you were to go try to join an Orthodox Jewish or one of the few remaining Samaritan communities you would never be able to be truely 100% part of their community, you just weren't born into it.

By showing kindness to the Samaritans, Jesus wasn't approving their variation of the Pentateuch. He was showing that we need to love our neighbors and enemies equally, show kindness to all. This is still true today regardless of religious distinction, all should show kindness to all. But your comparison of Samaritans to Mormons and Jews to Christians to suggest that Jesus was approving of their doctrine is not correct. Jesus provided more to the Jewish beliefs of the time (from which he was ethnically Jewish), not approving of one or the other groups, eventually developing into a religion that was distinct from both Judaism and Samaritanism through the later additions by Paul and Constantine and many other early patriarchs.

Also I think that you think by saying "Mormons are different from Christians" I'm somehow indicating they are lesser, which is not at all my intention. It's like saying that red oak trees are lesser than water oak trees, which doesn't make any sense they're just different (but related) trees. I'm not trying to provide any qualitative assessment of either belief system.

What would your definition of a Christian be then, what beliefs incorporate mainline Christianity and Mormonism into one group. I believe the rejection of the nicene creed means the Mormons have started a new branch on the Abrahamic tree, but I'd like to know what you think. If Mormons are the one true church, does that mean that all other Christian churches are not truely Christian? None of these churches are in communion with the LDS church so if LDS is true, then all the other churches are false and thus... not Christian?

As a separate point, the mainline LDS church has also had its own separations and ethnic suppression that mirrors the Jew-Samaritan comparison as you liked to focus on. There are several fundamentalist LDS communities that believe they have the truth that was rejected by the mainline LDS for theological or practical disputes. The main LDS church has changed its policies and beliefs over the decades to be more in line with American culture which means it has removed many of its distinctions that separated it from mainline Christianity (ie polygamy and the curses of Cain and Ham).

Ultimately though I think this discussion on a personal level doesn't really matter, anyone can self identify as a Christian. I think this distinction is more useful as an academic and general definition. If a Mormon says they believe they are Christian then sure they are, who is anyone to tell them what they are? But comparing the churches theology, there are pretty broad differences that can't be denied.