Clinton was a student of PNH, graduated Level 3, I've heard, before striking out on his own. So, he learned to use the cs under PNH.
From what I've read online, Pat was inspired to come up with the fiberglass stick while riding a certain horse bridleless in a public demo. I have no reason to doubt this story.
Regarding his mentors: I'm sure that neither Dorrance brother innovated the stick, nor used any kind of stick as the usual modus operandi, as Pat does (in Bill's book, True Horsemanship Through Feel, there's one case where he suggests using a stick, but he says to just find one in the woods!) Leslie Desmond, Bill's protege who's carrying his torch, also doesn't use any kind of stick. Pat's other mentors, Ray Hunt & Troy Henry, I also believe didn't normally use any kind of stick. So, I've concluded from these facts that Pat is the cs innovator & those who use fiberglass sticks with removable strings are imitators. Which is fine with me, because I think the cs a really useful tool for many aspects of hms.
The difference between a whip & a cs: the cs is used in both a friendly manner (to stroke the horse, keeping handler at a safe distance if needed) & in a directing manner (contact with horse & "driving", not contacting his body).
The handler should always regard the cs as an extension of his arm, because that's really what it should be (not a whipping tool, which sends a different feel to the horse).
Hope that helps!