This would be cool but I doubt it. They are probably new characters that just happen to look exactly like other characters from Advent Children, Nomura has done that kind of thing before in his games, designing very similar 1:1 characters that are not connected to each other.
It’s a story cutscene from the Beta. The darkness guys that look like Sephiroth can mind control people using feathers of darkness or something. Once the feather is removed from their body, they don’t remember anything they did or why they did it.