But did Saul even know? There’s no proof he did.
They'd absolutely charge him in a case that big. Doesn't matter if he knew or not. The Feds would have suspicion that he knew, and if he beat those charges they'd still have a dozen others to send him up the river on. They literally threw the book at him and charged him with every single thing they could, which is pretty accurate as to what would happen in real life.
On principle alone, the feds would charge someone like Saul in a situation like that. You gotta remember, Walt told the cops on the phone in Ozymandias that Hank crossed him, and that he was responsible for Hank's murder. It was a lie, but he said it to protect Skylar.
The Feds know that Walt and Saul both went on the run, and they assumed Jesse did too. That's all they need to know to charge everyone involved in Walt's enterprise with "accessory to murder after the fact". Being involved anywhere close to the murder of a federal agent is about as deep shyt as you can be in.
