Hi Vic. Am I bored? Of course not! These discussions help me to dig into the Bible, to know the truth.
Concerning 'the angels which kept not their first estate', it is my view they were the "sons of God' which saw the daughters of man (the Adam--Hebrew) that they were fair, and they took of them wives of all which they chose" (Gen. 6:1-2).
Details aren't given, but perhaps they entered into men, just as people became demon possessed, as in the days of Jesus on earth.
It's as if Satan sent them to try and contaminate the human race at that time, probably knowing that through the seed of the woman the serpent's (Satan's) head would be bruised (see Gen. 3:15).
I believe it is these angels God has cast into the 'caverns of gloom" awaiting their time of judging, as described in 2 Pet. 2:4-5 and Jude 6:
2 Pet. 2:4-5 "For if God did not spare the angels when they sinned, but cast them into hell (tartarus--Greek) and delivered [them] into chains of darkness, to be reserved unto judgment; and spared not the old world. but saved Noah the eighth [person], a preacher of righteousness, bringing in the flood upon the world of the ungodly." KJV
Jude 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." KJV
Whether or not these were Satan's angels, we are not told. If so, then I would assume they would suffer the same fate as Satan and his other angels.
If not, then it may be that after their judging, they will be allowed to return to their 'own habitation' in the heavens.
It is interesting that Paul says in 1 Cor. 6:3 "Know ye not that we shall judge angels?"
As to those "half man-half angels", one commentary states the belief that the flood came on because of these "giants". And that they would not be resurrected. I'm not sure if I believe that.
Bick