We agree and disagree.
At the beginning and the end of your post.
The Talmud shows us that the Rabbis have been debating the meaning of the Nephilim since the beginning.
I was never traditionally taught that the fallen angels were the Nephilim, and I was Baptist.
The fallen angels have always been held in the abyss until the end as per the letter of Jude.
The Nephilim have to be interpretted differently.
And therefore demons are not fallen angels.
That has to be rethought by people as well.
2 Peter 2:4 For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment;
Jude 1:6 And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.
A little background first to show these angels that are in chains of darkness are separate from the third that followed after Satan as they too rebelled against God leaving their first estate that was their proper place in heaven. These chained angels can not be the same angels as the principalities and power of darkness still in high places under the direction of Satan, Ephesians 6:12.
1 Peter 3:18-19 Jesus never went down to hell (lower parts of the earth) to preach as scripture says He went to preach to the spirits in prison. When Jesus laid in the tomb for three days his body was dead, but His spirit was very much alive. In 1 Peter 3:19 it says Jesus being put to death in the flesh, but quickened by the Spirit went and preached to the spirits in prison. The spirits in prison that Jesus went to preach to are those angels that are reserved in everlasting chains under darkness unto the judgment of the great day. Scripture does not say what Jesus preached to them, but some speculate that He was proclaiming to these fallen angels that Satan had no victory over His death. These angels are separate from the third Satan gathered with him as these angels bound in chains kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day, Jude 1:5-7. These angels were disobedient when once the longsuffering of God waited in the days of Noah while the ark was being built, 1 Peter 3:18-19. Big difference between these chained angels and the other ones we read about in Ephesians 6:12. Notice where it says they are reserved in chains meaning they are bound by God and can no longer cause man to sin.
These other angels being a third that oppose God under the leadership of Satan as being his messengers here on earth as they too are created spirits of God that is why God could never destroy any of them because a spirit can not die. (Matthew 25:41; 2 Peter 2:4; Jude 1: 6; Ephesians 6:12). We often call these types of angels demons/devils/evil spirits/unclean spirits for which an everlasting fire is prepared by God for Satan and these angels, Matthew 25:41.
The term demon comes from the Greek "daimon". Demons (fallen angels) are also called evil spirits, unclean spirits and devils in scripture as they are disembodied spirits as angels have no form. The use of the terminology of demons, evil spirits, unclean spirits and devils are interchangeable in scripture as they all mean the same thing. They exist in a demonic realm as principalities, Romans 8:38, 39; Ephesians 6:12, and have a hierarchy of rulership controlled by Satan as being his messengers, Matthew 12:22-30; Mark 3:22; Luke 4:35. They have no physical form, but can viciously posses a body, Matthew 8:16-31; 12:43-45. Demons are numerous in numbering and can also speak like the Legion (2000) of unclean spirits in Mark 5:9.
Strong's Concordance: Demon
Part of speech: noun masculine
Transliteration: daimon
Definition: a demon
Usage: evil spirit, demon