4. Judith Miller's source is/was in the CIA
This is the most likely option, I think - after Joe Wilson's July 6, 2003 op-ed was published, Miller surely probed her WMD sources for info and from her series on WMD she most certainly had one or more good sources in the CIA. Might have been Tenet himself, might have been some other(s). She might well have previously known of Valerie Plame from researching WMD issues, but If she did learn about Valerie Plame from a CIA source, even one no longer with the agency, she would be most anxious to keep it under wraps. The whole thing about the source signing a release that might be 'coerced' could just be a red herring if in fact her source was someone who could not be coerced in that way because he was now ex-CIA...... that's my take on it, fwiw.....
Interesting theory. Regardless, I'm convinced that the source is someone with an axe to grind against the Bush Administration. The protection is being provided because the public will recognize the sleaze at work if the identity is disclosed. Miller certainly isn't falling on her sword to protect a Bush loyalist.