Libby talked to Russert after he learned about Plame from several sources, not before. It is a timeline thing.
We only have access to the grand jury testimony that makes Libby look guilty. Libby is a lawyer. He was not likely to lie in a way that he could easily be caught.
It works like this. How do Cheney and Libby know what the other is going to testify to. Cheney and Libby each meet with two lawyers separately. One of the lawyers is representing Cheney the other is representing Libby. The Cheney lawyer asks Cheney questions while Libby's lawyer listens. Then the two lawyers meet with Libby and Libby's lawyer questions while Cheney's lawyer listens. Then both Cheney and Libby can say they never talked to each other about their testimony and the two lawyers can say they have only questioned their own client. But Libby knows what Cheney is going to say, and cheney knows what Libby is going to say. The two lawyers make corrections with their clients to get the stories straight.
That is what happens in the real world.
If I have been involved in using such tactics then surely defense lawyers like Libby have used them too. I can't imagine administration people going in whith out being aware of what the other adminstration people are going to testify.
Libby is not accused of revealing who Valery Plame was. He is accused of lying about his sources and what he did or didn't tell them. For that to be true the sources he listed had to not be sources or he had to not name some actual sources he was questioned about.
That is the rational behind the perjury, lying to an officer and the obstruction charges. The time line is not important. Proving Libby lied is important for a conviction.
Proving that A told Libby before B told him does not prove Libby was lying when he said B told him.