I'll bet you're right on this. Given the knowledge requirement under the statute he may have concluded some time ago that even if her name and status was disclosed by an administration person there is not sufficient evidence that a crime was committed.
I gather he thinks somebody has testified falsely to the grand jury, and that Miller's testimony can prove it.
Agreed - either that somebody has testified falsely to the grand jury, or that somebody has obstructed justice by suborning or attempting to suborn the false testimony of someone else who has been or might be a witness in this case.
The only way that could be true is that the testimony concerned something said to Miller, either by the witness or by a third person in his presence. I say something said to Miller, not by her, since could not have had an obligation to withhold what she herself said.
Disagree here - I would bet that the position Miller's and the NYT's lawyers are taking as to First Amendment protection is significantly broader than you suggest, and goes not only to the statements made by a source in an interview but also the statements put to the source by the journalist, in fact that the entire conversation is protected on freedom of the press grounds.
As to speculation. I am highly skeptical that Miller has been languishing in jail for the sole purpose of protecting a republican politician like Libby or Rove. My guess is that what she is really concerned about is that her statements made to a grand jury concerning what she and Libby discussed might contradict what other journalists have given evidence as to, and that either she or the other journalist might be exposed to a perjury charge. This might have arisen, for example, if for some reason she told another journalist something different about what she and Libby discussed than what they really discussed. The testimony of the other journalist would then result in a discrepancy between Libby's own testimony and the version related by the other journalist of Miller's conversation. Why would Miller have retailed to her journalistic colleagues a different version of the conversation than what actually happened? Perhaps because she was attempting to portray Libby in a bad light, for example claiming that Libby told her about Plame being a CIA agent, when in fact she knew it all along. And why not simply tell the grand jury the same false version she told to other journalists and leave Libby out to dry? It must be that Fitzgerald can prove that that version is false and that Libby's version is the truth (e.g., by demonstrating that Miller already knew of Plame's status through her direct relationship with Wilson or Plame). The foregoing is speculation on my part.
All right, I concede the point.
This might have arisen, for example, if for some reason she told another journalist something different about what she and Libby discussed than what they really discussed.
That doesn't make a lot of sense; a contradictory statement out of court is not nearly enough to support a perjury charge.