Is there any benefits or difference from a fully charged sleep and lethargy as oppose to a quickly charged one?
With lethargy the longer you charge the better the aim assuming you move the cast area around to where you need it. (I don’t remember if sleep is the same but I decided I didn’t like that skill very much so I don’t use it.) I don’t know if there is any other benefits other than that unless your asking about skill secrets.
No. There isn’t a cast time for both skill.
It’s probably a placeholder for you to aim your skill.