I believe Lyndon Johnson did a lot of recording. I remember a few years ago one of the morning news shows a few years ago had a recording of LBJ talking to Jackie Kennedy where he almost seem to be flirting with her (LBJ was quite the ladies man). Ironically when Nixon became President, he had the tape recording equipment removed but a couple years later installed his own system.
Depends on what you mean by “threats”. “If you take such-and-such a stance, I will have to break with you” or “. . .I will oppose you” is a threat, but I would have though not an improper one. I would thinks threats like this are made all the time. “I will break your legs/your children will have a nasty accident on the way home from school” is a different kind of threat and I doubt that it’s made very often in private conversations between politicians.
Conversation is businesslike, and usually polite. If not polite, it’s “firm” or “frank”. It doesn’t get down to attacks and abuse very often, I would think, if only because the conversation wouldn’t happen at all unless both parties hoped to achieve something from it, and not much is likely to be achieved in a shouting match.