I'm not sure these additional beliefs necessarily disqualify them as "Christian" though as they do appear to have retained most of the fundamental basics.....
I suppose it depends on ones definition of Christian. No doubt plenty of "Christian" denominations consider that others aren't.
"In truth, there was only one Christian, and he died on the Cross" -Nietzsche
Most mainstream Christians don't consider Mormons as Christians because they have added to the Bible, which is considered a heresy.
[1] https://en.wikipedia.org/wiki/Nicene_Creed#Comparison_betwee... (note that "catholic" in this means "diverse" and doesn't specifically refer to the Catholic church)
[2] the differences between the two creeds listed there are basically what split the Orthodox church from the Western church and indirectly led to the fall of Constantinople. So some people consider them fairly important, but others don't.
That was never a christian belief to begin with.
The atonement was (supposedly) because of God's own incarnation as Christ, our of love, not because of some blood offering to God (i.e. the crucifixion, which was not even meant as such, anyway).
Romans3:25, Hebrews9:22, Rev1:5 for instance. There are plenty more. Very prominent concept. Which is why up until recently there was plenty of talk about being "washed in the blood" in Christian churches.
(I believe the manifest brutal cruelty and outright ridiculousness of the concept was one of the primary things leading to my rejection of Christianity at a very early age.... no offense to anyone's particular faith... just my point of view)
I believe the Biblical narrative describing the significance of Jesus' sacrifice was about past sins, that which humans bore without actually committing. I do not believe the death of Jesus in any way suggests, in Christian folklore, that you don't need to atone for sins.
FWIW, I think the Catholic Church (which didn't exist until hundreds of years after the whole Jesus thing) considers things like rosary prayers as atonement.
I realize the idea is that the blood of Jesus was supposed to end blood atonement and usher in a new era. However, I never thought this idea was reasonable, never identified with a God who would set such a system up, and never felt that this idea at the core of a religion would ultimately lead to positive outcomes or really much more than continued barbarism and cruelty, but I'm not going to argue with your faith.
Given what he had to work with I think he did pretty well.
Mormons do get drawn outside of Christianity by some christians for a variety of reasons (non-trinitarian, open canon, cosmological background quasi-polytheism), and some people have suggested classifying the faith as another Abrahamic one as a means of sidestepping that discussion, which may be where one could get the idea that the comparison to Muslims is a good one. But if Mormons are outside mainstream Christianity, it's only marginally so.
They are as Christian as catholics are, perhaps even more so as in my opinion they follow his ideology a whole lot better.