The unmasking and interview were all legally justifiable and necessary. Flynn worked on a campaign that had received assistance from the Russian state, which Obama, McConnell, and Ryan knew at the time but McConnell refused to support any statement acknowledging that in October. Under the normal order of business, the government decided to find out who the Russian ambassador was talking to discussing removing sanctions, etc. Then, Flynn was secretly working for the Turkish government while receiving classified intelligence reports from the US government. He also apparently sat in on a plot to kidnap and render Fethullah Gulen from Pennsylvania.
The whole drama over Flynn from the right is kayfabe bullshit. It's Barr and Trump putting their thumbs on the scales so that Trump can use this to distract from his incredibly disastrous handling of COVID-19 and to use against Biden in the election. The justification that DOJ is using isn't afforded to anyone else, and they misconstrued or lied about the information provided by others involved to portray it as a miscarriage of justice. Barr is the most dangerous man in the Trump administration. He was responsible for HW Bush pardoning everyone involved in Iran-Contra (to protect HW) and has had a hand in the corruption of USDOJ to protect Donald Trump. Barr is the most dishonest, corrupt Attorney General in US history, which is impressive given he worked with one of the others (Edwin Meese).