You have said that angels can take human form, but demons need to possess a human. How then,were they able to procreate with women to produce the nephilim?
In Hebrews 13:2 the Bible mentions that people have entertained angels thinking they were human, so we know they can take on human form. Genesis 6:1-4 speaks of certain angels marrying human women and fathering hybrid children called nephilim with them. Jude 1:6 says these angels abandoned their original form, indicating they took on human form to accomplish this.
But there is no indication from the Bible that demons have the ability to independently take on physical form although they long to do so. Matt. 8:28-30 records demons begging Jesus to send them into a herd of pigs instead of just driving them out of the men they were possessing. Matt. 12:43-44 says when an evil spirit comes out of a person it searches for another person to inhabit. (According to Mark 7:25-26 evil spirits and demons are one and the same.)
Based on these references, I’m convinced it’s not accurate to think of fallen angels and demons/evil spirits as being the same.