It is an interesting hypothesis, but one that fails on a number of points:
1. According to most commentaries I have read, Nephilim of the Old Testament were not fallen angels, but the offspring of the "sons of God" and the "daughters of men" (Genesis 6:4).
2. There are good reasons to believe that the name Nephi may be of Egyptian origin. See the article Internal Textual Evidence for the Egyptian Origin of Nephi's Name.
3. As we discussed earlier, the angel who delivered the plates to Joseph Smith was Moroni, not Nephi.
4. Three individuals named Nephi are featured prominently in the Book of Mormon. None of them is described as being a fallen angel; none fits the description given in Genesis 6. All three were prophets of God who bore strong testimony of Jesus Christ.
5. For me, the best argument against your hypothesis is provided by the Book of Mormon itself. The book's purpose, as stated on its title page,:
. . . is to show unto the remnant of the House of Israel what great things the Lord hath done for their fathers; and that they may know the covenants of the Lord, that they are not cast off forever— And also to the convincing of the Jew and Gentile that JESUS is the CHRIST, the ETERNAL GOD, manifesting himself unto all nations—And now, if there are faults they are the mistakes of men; wherefore, condemn not the things of God, that ye may be found spotless at the judgment-seat of Christ.
The rest of the book does indeed testify with great power of Jesus Christ. I submit to you that such a message would be a strange one for a fallen angel to proclaim.
3. The Hebrew-Greek Key Study Bible, Spiros Zodhiates, Th. D.
“The term ‘giant’ from the Hebrew NEPHILIM (5303) is influenced by the Greek term GIGANTES in the Septuagint. Nephilim comes from the verb NAPHAL (5307) meaning “to fall” in general, but is often associated with violence, hence often translated ‘overthrow, fall upon.’ In Numbers 13:32,33 it is also noted that they were ‘men of great stature.’ Emphasis should be placed upon the fact that they were men of violence who had no respect for other men.”
The scholars of the NIV presume it is impossible for angels to have physical bodies. I challenge that assumption. There could be more spiritual paths and creations in heaven than we assume. Let’s also suppose that angels who enter our fallen world can become mortals because of sin, which sin causes death. According to the Bible, then, the “sons of God” or angels could logically fall. And thus, Nephilim, or the name of their children, would be a combination of “Naphlu”, meaning “they fell”, and “hipilu”, meaning they caused the downfall of the world. Interestingly, this interpretation of the word “Nephilim” is given by by Charles B. Chavel, a Jewish Rabbi, in Ramban (Nachmanides) Commentary of the Torah. Although Chavel does not agree with my view on fallen angels, his interpretation of the word Nephilim fits perfectly. Of the six sources I quote, I believe the Jewish mind is a more credible interpretation.
The Nephilim had contaminated the gene pool, and they were spreading their false doctrine of spiritual genetics and spiritual evolution on earth, and therefore, God needed to send the flood to cleanse it. This view also explains why “giants” such as Goliath and others are mentioned in the Bible after the flood. It is probably that some of the genes were passed on through some of Noah’s children or through Ham. The Nephilim had so dominated the gene pool, that even after the flood, the genetic influence was still felt by human beings. (2 Sam 21:16-22; 1Chr 20:4-8; Job 16:14; Num 13:13; De 2:11,20; 3:11-13; Josh 12:4, 13:12, 15:8, 17:15, 18:16) Thus, the interpretation of immortal angels falling into sin and death gives a clearer explanation of genetic origin of the giants mentioned in the Bible repeatedly. Do we have a better explanation anywhere?
The view of fallen angels in Genesis 6:1-5 is not from my own limited ex-LDS perspective, but it has been in the mind of scholars for thousands of years and this view is recognized by Bible commentaries such as the notes in the NIV Study Bible:
This is from another website that shows a different view and does NOT contain Church of Jesus Christ of Latter Day Saint propaganda.
http://www.nccg.org/nefilim/ch25.html