The simple biblical definition of what is a Christian can be found in Matthew 16:15-17
15 He said to them, “But who do you say that I am?” 16 Simon Peter replied, “You are the Christ, the Son of the living God.” 17 And Jesus answered him, “Blessed are you, Simon Bar-jonah! For flesh and blood has not revealed this to you, but my Father who is in heaven.
While Evangelicals would like to add all sorts of additional requirements to those given by Christ himself, by this straightforward biblical definition, Mormons are most certainly Christians.
This definition, by the way, also has the additional benefit of distinguishing Muslims from Christians; Muslims accept Jesus as the Christ/Messiah, but reject the idea that he is the Son of God.
So, it seems that you are asserting that profession of Jesus as the Christ makes one a Christian. It seems to me that in Matthew 7:21-29 Christ himself defines who His disciples are, so to me a Christian is one that “heareth” and “doeth” His doctrine. If that’s correct then I would answer the question “Are Mormons Christian” with the answer some are pretty good Christians and others are pretty poor Christians and I’ve give the same answer to the question “Are Methodists Christian”.
There are really two questions that need to be part of this debate anyway, in my opinion. Not only “Are Mormons Christian” but “Are Mormons Authorized to act in Christ’s Name and with His Authority?”, because these two questions have different answers.