[MAGIC] In Lord of the Rings, there is a distinction between Elvish 'magic' and normal magic. How did JRR Tolkien's belief influence this lore? How would other Christian reacts if he chose to portray magic otherwise?

Tolkien goes into this fairly in depth in a letter he wrote to Milton Waldmon in 1951. You can find this letter in the introduction to the second edition of the Silmarillion. The general gist of the difference is that elven "magic" is primarily intended to preserve things as they are, and that "evil" magic is designed to coerce or control others.

Tolkien outlines that, as far as his work has "themes" that he is willing to admit (he famously dislikes allegory), the themes can be rendered as "The Fall, Mortality, and the Machine".

I will quote a large section now about The Machine:

"By the last I intend all use of external plans or devices (apparatus) instead of developments of the inherent inner powers or talents – or even the use of these talents with the corrupted motive of dominating: bulldozing the real world, or coercing other wills. The Machine is our more obvious modern form though more closely related to Magic than is usually recognised.

I have not used ‘magic’ consistently, and indeed the Elven-queen Galadriel is obliged to remonstrate with the Hobbits on their confused use of the word both for the devices and operations of the Enemy, and for those of the Elves. I have not, because there is not a word for the latter (since all human stories have suffered the same confusion). But the Elves are there (in my tales) to demonstrate the difference. Their ‘magic’ is Art, delivered from many of its human limitations: more effortless, more quick, more complete (product, and vision in unflawed correspondence). And its object is Art not Power, sub-creation not domination and tyrannous reforming of Creation. The ‘Elves’ are ‘immortal’, at least as far as this world goes: and hence are concerned rather with the griefs and burdens of deathlessness in time and change, than with death. The Enemy in successive forms is always ‘naturally’ concerned with sheer Domination, and so the Lord of magic and machines; but the problem: that this frightful evil can and does arise from an apparently good root, the desire to benefit the world and others – speedily and according to the benefactors own plans – is a recurrent motive.”

There is definitely a strong christian theme running through LOTR, as I said above "The Fall" is extremely important in Christian thought. Its relation to magic in LOTR is also laid out in this letter:

“his desire is at once wedded to a passionate love of the real primary world, and hence filled with the sense of mortality, and yet unsatisfied by it. It has various opportunities of ‘Fall’. It may become possessive, clinging to the things made as its own, the sub-creator wishes to be the Lord and God of his private creation. He will rebel against the laws of the Creator – especially against mortality. Both of these (alone or together) will lead to the desire for Power, for making the will more quickly effective, – and so to the Machine (or Magic).”

Hope this helps. I don't think you can really answer your last question, so I will leave this here.