Floyd Landis is neither offence nor defense in any criminal process. He is a witness. His role is simply to provide information to the investigators and to present testimony if/when required. To use another aged sporting analogy, he is standing on the sidelines and doesn't get to handle the ball.
As for any civil process, why sue Armstrong for suggesting that Landis is mentally unstable? I doubt Landis can establish that Armstrong's comments to the media had any serious deleterious effect on his income, livelihood, or reputation. There seems to be no return, and it doesn't seem to be the focus of Landis's attention right now.
As for defending against any civil/criminal action, I highly doubt the government is coming after Floyd Landis. Whistleblowers are not usually devoured, it makes it hard to solicit information in future. I don't know if he committed any perjury in his doping case, whether or not his testimony was sworn and whether or not he denied any drugs other than synthetic testosterone are unknown to me. Nowitzky is not on Landis's side nor against him. He is merely an investigator, independent of any witness. His role is to attempt to uncover evidence of any criminal act and present a case to the prosecutors. Landis will be one of potentially many witnesses.
Can Nowitzky tape phone conversations? Well no doubt the laws of California and other states have provisions for interception of private communications, either unknown to both parties or with one party consent. Not all offences are relevant to this invasion of privacy in my country (not sure about Cali), but if there is trafficking or a conspiracy to import/distribute a controlled substance then it will likely be applicable.
What kind of tasks are Landis's lawyers doing? Probably reviewing all matters that could prove damaging to their client and providing advice and/or taking necessary steps to protect him. And probably (hopefully!) nothing to do with the federal investigation outside of advising their client.