If Fitton knew of the documents, and encouraged their retention, is he an accessory?

Probably not. Typically to be an accessory you have to have done something to help in the commission of the crime or have done something unlawful to help them avoid getting caught (accessory after the fact). Typically, you have to do something.

Here's the federal version:

"Whoever, knowing that an offense against the United States has been committed, receives, relieves, comforts or assists the offender in order to hinder or prevent his apprehension, trial or punishment, is an accessory after the fact."

law.cornell.edu: 18 U.S. Code § 3 - Accessory after the fact | U.S. Code | US Law

Merely knowing about a crime usually isn't enough to be an accessory. A lot of the time, you don't have an affirmative duty to report crimes. Note - while that's the general rule, there are a non-trivial number of situations where you have an affirmative duty to report a crime (often when you have some duty of care or responsibility for a victim), and there might even be criminal sanctions if you don't. But that's usually not an application of the "accessory" crime, but rather an independent violation of a statutory obligation.

Giving your opinion about whether something is or is not legal isn't likely to be a crime either, unless you cross the admittedly vague line between discussing the issues and actually helping them in committing the crime. Doesn't sound like that's what Fitton actually did.