That's where I deviate. In the idea of nephilim and fallen angels being physical.
I believe we were all spirits before and that Satan and his angels (spirits) were kicked out when Christ rose to heaven. They don't have bodies which is why, as you mentioned, found possessing bodies of God's children.
I don't believe God would resurrect a person and allow them to fall and visit earth. Only the worthy are resurrected until the end in the second resurrection where some will suffer eternal damnation in resurrected bodies. And they are bound in hell.
When we die we are forced into hell or accepted into heaven. No other way around it. Which shows God's lack of allowing angels that are fallen to mingle other than Satan and his spirit angels.
We do read Satan is cast out of heaven with his angels down to the earth.
I assume some of these angels are still here causing trouble. Some are locked up by God. The scripture in Hebrews does not distinguish which kind of angel can appear as a man to men, I assume they still have this ability fallen angel or holy angel.
A fallen angel would appear to men to deceive them away from Christ and God.
7 And war broke out in heaven: Michael and his angels fought with the dragon; and the dragon and his angels fought, 8 but they [
a]did not prevail, nor was a place found for [
b]them in heaven any longer. 9 So the great dragon was cast out, that serpent of old, called the Devil and Satan, who deceives the whole world; he was cast to the earth, and his angels were cast out with him.