As for the first one, I think the reason was that Navalny called him after the investigation and clearly showed at the beginning of the conversation that he kne

As for the first one, I think the reason was that Navalny called him after the investigation and clearly showed at the beginning of the conversation that he knew a lot of secret things, including the surnames of the other agents, facts on when and where they were, etc. Probably, the FSB guy just couldn't believe that there was a chance that a person who doesn't work for the government could be so aware of the details of the operation.