The word son, in the phrase "sons of God" characterizes “of the same group”, nothing more and nothing less.
Job 1:6 Now there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them.
Seth is simply reckoned as one of the many that can be used as an outward representive of that not seen (the spiritual seed by which all men are born from above, Christ...that make up the spiritual incorruptible seed, Christ.This is according to the spirit not seen, and not the flesh as human seeds, seen.
alatians 3:16 Now to Abraham and his seed (Christ) were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
Flesh give birth to flesh. The Spirit of Chrsit gives brith to a new eternal spirit that will never die .
John 3:6 That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.
For as many that are led by the Spirit of Christ are sons of God.
Some of those take are led fall away crucifying the son of man over and over as if one demonstration was not enough and Christ must be recruicifed over and over. Being led by the Spirit of Christ they refused to follow at that point.
For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,And have tasted the good word of God, and the powers of the world to come,If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame. Heb 6:4
The angel (messengers) are not subject to salvationare not included. They who have no form (sinful flesh )are not subject to the gospel .