Although I have been reluctant to debate the heterodoxy of Mormon theology, this is not because it is intrinsically difficult to show that Mormon theology is compatible with mainstream Christian theology - it is because almost nobody is actually interested in showing this harmony.
Mainstream Christians are (almost always) concerned to show that Mormonism is heterodox and beyond the pale, while Mormons are generally happy to acknowledge fundamental differences such that restoration of the gospel can be shown to have been necessary.
Therefore both Mormons and Mainstreamers bring to the task the assumption of incommensurable theological differences - and with that assumption it is trivially easy to find incommensurable theological differences.
But bringing to the task, as I do, an assumption that differences are superficial and mask a deeper harmony, then it is easy to discover harmony.
The key is to recognize that Mormon theology is concrete, personal and simple - such that it can all be fully understood by the average eight year old; and armed with this principle (and with an assumption of harmony) it can be seen that when Mainstream and Mormon appear to diverge this can be seen to be superficial only.
In fact this isn't at all difficult to do! (Else I would not myself be able to do it; since I am not a deep theologian and am indeed impatient with theology.)
For example, Mainstream Christians say that God created everything from nothing, while Mormons say that he created form from chaos and that there never was 'nothing'.
So for Mainstream 'the void' is nothing, but for Mormons is it formless 'stuff'; matter and energy and the rest of it.
But the Mormon view simply recognizes that humans cannot think about something coming from nothing; but can imagine God as a sculpting the world from eternally existing stuff.
Or, the contrast between Mainstream Christians saying that after death, humans - which are not gods - are (potentially) adopted to become Sons of God, and above the angels and adopted brothers of Christ; and the Mormon belief that humans are the actual spirit children of God with Jesus as an elder (and higher) brother, who have volunteered to be clothed in bodies for mortal life to learn important lessons, then (if they pass the tests) potentially returning to live with God at a higher spiritual level and in perfected bodies after death.
The Mormon concepts can be seen as explaining how it is that we could become what Christ promised - Sons of God. If (on the Mainstream view) we are not already divine then since 'adoption' seems too weak to make us divine, because adoption would seem to leave our essential natures unchanged (in this world, adopting a boy is a matter of granting them the rights of a son but not of changing their essence).
But if we were already divine sons before coming to earth, then it is all understandable.
Of course, to go along with this style of explanation requires an acknowledgement of the inadequacy of Mainstream Christian theology - on the basis of 'if it ain't broke, then what is the point of fixing it'; to be sympathetic to the rise of Mormonism one has to feel that Mainstream Christianity is, at least for some people, 'broke', inadequate, ineffective.
This broken-ness seems obvious to me (as evidenced by chosen sub-fertility, to go no further with the evidence).
And one has to be unhappy with the abstractness of what purport to be mainstream 'explanations' - such as attempts to explain the Holy Trinity.
Examples of attempted explanations would include the Athanasian Creed:
And the Catholick Faith is this: That we worship one God in Trinity, and Trinity in Unity; Neither confounding the Persons: nor dividing the Substance. For there is one Person of the Father, another of the Son: and another of the Holy Ghost. But the Godhead of the Father, of the Son, and of the Holy Ghost, is all one: the Glory equal, the Majesty co-eternal. Such as the Father is, such is the Son: and such is the Holy Ghost. The Father uncreate, the Son uncreate: and the Holy Ghost uncreate. The Father incomprehensible, the Son incomprehensible: and the Holy Ghost incomprehensible. The Father eternal, the Son eternal: and the Holy Ghost eternal. And yet they are not three eternals: but one eternal. As also there are not three incomprehensibles, nor three uncreated: but one uncreated, and one incomprehensible. So likewise the Father is Almighty, the Son Almighty: and the Holy Ghost Almighty. And yet they are not three Almighties: but one Almighty. So the Father is God, the Son is God: and the Holy Ghost is God. And yet they are not three Gods: but one God. So likewise the Father is Lord, the Son Lord: and the Holy Ghost Lord. And yet not three Lords: but one Lord. For like as we are compelled by the Christian verity: to acknowledge every Person by himself to be God and Lord; So are we forbidden by the Catholick Religion: to say there be three Gods, or three Lords. The Father is made of none: neither created, nor begotten. The Son is of the Father alone: not made, nor created, but begotten. The Holy Ghost is of the Father and of the Son: neither made, nor created, nor begotten, but proceeding. So there is one Father, not three Fathers; one Son, not three Sons: one Holy Ghost, not three Holy Ghosts. And in this Trinity none is afore, or after other: none is greater, or less than another; But the whole three Persons are co-eternal together: and co-equal. So that in all things, as is aforesaid: the Unity in Trinity, and the Trinity in Unity is to be worshipped. He therefore that will be saved: must thus think of the Trinity.
The Father incomprehensible, the Son incomprehensible: and the Holy Ghost incomprehensible... He therefore that will be saved: must thus think of the Trinity.
In which case, how may anyone be saved?
(Let alone the children, who Christ assured us would be saved).
Of course, I am being mischievous, but I have studied many, many descriptions of the nature of the Holy Trinity from Eastern Orthodox, Roman Catholic, Anglican and Conservative Evangelical theology - and I find all of them incomprehensible (if I am honest).
Yet I understand the Mormon description of The Godhead, and so would most children. From lds.org: Teachings/ Gospel Topics
Or, from Articles of Faith by James E Talmage 1890 (1962 edition).
Three personages composing the great presiding council of the universe have revealed themselves to man: 1. God the Eternal father, 2. His Son, Jesus Christ, 3. the Holy Ghost. That these three are separate individuals, physically distinct from each other, is demonstrated by the accepted records of divine dealings with man... The Godhead is a type of unity in the attributes, powers, and purposes of its members... The unity is a type of completeness; the mind of any one member of the Trinity is the mind of the others; seeing as each of them does with the eye of perfection, they see and understand alike. The one-ness of the Godhead... implies no mystical union of substance, nor any unnatural and therefore impossible blending of personality. Father, Son and Holy Ghost are as distinct in their persons and individualities as are any three personages in mortality. Yet their unity of purpose and operation is such as to make their edicts one, and their will the will of God.
While Mainstream Christians see this as clearly heterodox, indeed heretical; I see a clear and comprehensible explanation of the Holy Trinity which does the job - something that Mainstream definitions fail to do.
And 'The Job' is to enable us to have a personal relationship with God in His three persons, to understand God's character, motivations, intentions, emotions and so on - so that even a child can live in communication with God as Father, Brother, and Protector/ Comforter/ Teacher.
Faith is Trust; and we can only trust a person - not an abstraction. Thus the value, and perhaps (for some people) the necessity of the kind of concrete, personal and simple version of Mainstream theology which Mormonism provides.