No, Mormonism isn’t Christianity.
Why? Because their doctrine departs from fundamental Christianity.
Using the laws of logic, if something is true, then all other possibilities are false. Truth is exclusive.
Next question.
IMHO no church can guarantee you heaven, and no church can deny it either. Jesus looks on the heart and will make sure that all those he calls his are saved. It's one of the reasons Mormons perform the Biblical ordinance of baptism for the dead.
(Before anyone gets bent out of shape, if you accept the atonement of Jesus Christ, you accept the principle of proxy work)