Panel 3: Is that some sort of way for the sword to swivel?
Late reply, but it looks like a hex-nut assembly going through the edge of the habaki. I'm not sure what purpose it could possibly serve, but maybe it somehow secures the habaki to the tang of the sword.
Most likely, it was just a cool little industrial detail Imizu decided to put on the sword to make it look more tactical.
Hmph!Don't go thinking that rushing at opponents head on works all the time.