I think you can't consider demos like videos.
I haven't used cl_freezedemo so I don't know how it could be used. Could be that was an attempt to make something useful, but is not
As for the rewind, I imagine playing a demo is reproducing the events contained in your demo file, and letting the engine doing its work. That means : the engine can shoot a rocket right in front of you, but it can't make it go back.
What you're asking for is more a "post production" job, I mean, you should use some video editing tools to make pauses and rewind and such
Edit: and oh, maybe i'm all wrong
(and you still are right demo playback could be improved, for some kind of rewind maybe without producing video while rewinding for expl)