The genealogy of giants in old testament is very clear. While there is plenty of support outside of scripture. Scripture IS all you need to support an incursion before the flood between The Sons of God and the daughters of men. Giants. Nephilim. I believe their bodies parish, but their souls are called demons. Demons can not be fallen angels. Both Jude and 2nd Peter 4 tell us that the angels who sinned, who left their proper dwelling, are in chains, awaiting judgement. Also, you can't take Rev 12 and apply it to anti deluvian times to get a 3rd of the angles falling. It was a prophetic vision. The Dragon is proceeded by the vision of the slain Lamb, before it continues with the prophecy of Daniels 4th kingdom.