Novak's testimony has nothing to do with what the prosecutor wants to talk to Cooper and Miller about. The prosecutor wants to know the substance of other conversations that the reporters had with the leaker on other subjects (per Fox news) . I heard, months ago that there was a collateral investigation which grew out of the Plame investigation, regarding a leak from the CIA to reporters, warning a terrorist supporting group of an imminent raid on their premises just hours before it happened. My guess is that the prosecutor is going after the CIA agent and it is this source that the reporters are protecting.
Novak's testimony (whatever it may be), and perhaps other evidence you mention, is the checkmate to Miller & Cooper simply walking in there with the same story - and lying.
If the truth indeed points to this collateral investigation - so be it, perhaps it will. Let them talk so we can find out.