the Logan Act aspect of this was always very weird to me. a 1799 statute that has never been used to prosecute anyone in history is not really a credible threat.
On the other hand it looks like Flynn very likely perjured himself when he talked to the FBI and that’s 5 years unless he rolls and says who sent him to talk to the Russians about sanctions.
He should not have lied (if he indeed did) but if so I think it was because he was afraid his Russian contact would be mischaracterized. But if he lied he only made it worse for himself.