Listening to foreign entities is legal without the FISA warrant but divulging or revealing the portion of the conversation with an American citizen is not legal without the FISA warrant.
That is why the FBI or DOJ are involved in getting the FISA warrants because they involve domestic investigations and surveillance.
Flynn's problem was not that he talked to the foreign ambassador but that he lied about the conversation to Pence, which got him fired.
The real crime in the Flynn caper, was that someone leaked the "unauthorized" portion of the conversation between Flynn and the Russian ambassador to the Washington Post.
That is a felony and that crime should be pursued and investigated.
Have I stated this correctly? Please comment if I am off base.
1) You are essentially correct unless those “foreigners” happen to fall under the broad meaning of “US Person”, which includes a whole range of entities that may not be US citizens.
2) Again, essentially correct, but the proceeds (internals and externals) are classified and subject to national security laws and regulations.
3) Correct. The Bureau submits a FISA affidavit to DOJ that documents the probable cause to initiate electronic surveillance (not just “wiretaps”) against the target, which may be a US Person. DOJ then requests a FISA warrant from the FISA court, which consists of 12 (I think that’s the current number) specially-appointed FISA judges.
4) That is not the only crime. It is highly unusual and probably unprecedented that the incoming National Security Advisor - a person who has had TS/SCI access and undergone polygraphs and Special Background Investigations every five years of his 30-plus year career - would be the target of a FISA warrant, nor would a FISA judge approve any such warrant. If the electronic surveillance was done by any agency absent a FISA warrant, ALL of them are governed by special laws, rules, regulations and community directives when encountering a US Person who is not a target of the investigation in the course of routine collection, e.g., collection against a foreign ambassador.