Subsequent to writing my article today, "The FBI's Flynn Interview Was Not Legitimate," I had the opportunity to read the now released "original" FD-302 that agents Strzok and Pientka wrote after their interview with Flynn.  Bear in mind that this version probably was run past Andrew McCabe before it was finalized.  Bear in mind that, per Mueller's letter to Judge Sullivan, Andrew McCabe appears to have helped shape this version in the "deliverative" process, even though he wasn't present at the interview and the interview wasn't recorded.

My reading of this FD-302 is that nothing that was discussed was in fact material to any legitimate FBI concerns or duties. 

It simply is not the FBI's job to interview every national security or foreign policy official of the US government after they have a contact with a foreign official in order to test their memory or even their truthfulness. 

The FBI has access to electronic intercepts of all those conversations.  If agents have any counterintelligence concerns arising from such contacts, they can open a counterintelligence investigation.  It's clear that this did not happen in this case.

The interview was simply an attempt to elicit statements from Flynn that could be interpreted as contradicting the official record – i.e., the recording of the conversations. In other words, it was a setup, and, as such, the case should be dismissed because the agents weren't conducting legitimate FBI business.