...it appears as though his curse seal might have had something to do with his breaking free?

That's the only variable that I can see allowing Sasuke to do something that Kakashi couldn't.
I can't remember what all the methods for breaking genjutsu are, but perhaps the seal passively inflicts a degree of pain that can break through?

And again, at this point I'm hoping that like Daryl just suggested, this has all just been for show to fuck with Zetsu's head, then they'll tag-team kill his flytrap azz, and everything will be hunky dory.

Cue the "The Bewlay Brothers".