* The word "Nephilim" is in English language Bibles in Genesis 6. By telling us that word is not Biblical, you are telling us that my English language Bible is not a Bible.
* The word "Nephilim" is written in the English alphabet. The ancient manuscripts of what we call the Old Testament were never written in the English alphabet. They were written in the Hebrew alphabet.
* The word "Nephilim" is a transliteration from the Hebrew alphabet to the English alphabet. It does not form a new word. It is simply an additional method of writing a Hebrew word, which remains the same, in another language.
* The rules for transliterating from one alphabet to another may allow for more than one spelling. I lived in South Korea for 35 months and I got to know the rules for transliterating from the Koren alphabet to the English alphabet, [NOTE: Contrary to what some may think, South Korea does have an alphabet. It actually has two systems of writing.] South Korean words had two symbols that could each be transliterated into two different English letters. One example is a South Korean letter that has been transliterated as either an "R" or as an "L." One is considered to be more correct, but both have been used.
* The Hebrew word transliterated as Nephilim can also be transliterated as Nefilim, but this is not common.
* While I could be wrong, I suspect that your listing of the Nephiyl/Nephil simply another form of the word, as "boys" is another form of the word "boy." Or, it could be an alternate spelling as I stated in my comment just above.