Who is the Son of Man?
Tarzan.
If Jesus was the only Son of Man, why would he say “it will fill the skies”?
It. Not “they”. It being the “blazing” arrival - an entry with lots of firework.
And why would he say it will happen before his return?
He does not say that, at least not in either of the passages you quote. Or is the concept of a dramatic pyrotechnic showman entrance where there are lots of (in this case, magical) pyrotechnics before the showman actually reveals himself so hard to grasp.
Why did John’s vision in Revelation describe the Son of Man […] carrying a symbol of 7 stars?
Because Abrahamic religion is absolutely obsessed with the number seven, and because there are seven celestial objects that are visible to the naked eye and are visibly moving.
Because the Son [sic] of Man are Pleiadians. Jesus was an incarnated Pleiadian Soul in a human body.
The Seven are just the ones that are visible on Earth to the naked eye - and even then, Pleione, being a variable star, is not always visible, so to some cultures, the asterism only has six stars, and in ideal conditions, up to nine stars may be visible. In truth, however, the Pleiades cluster holds over four hundred stars.