I actually say this quite a bit haha. As with most nuances in language, it depends on your tone. But the way I use typically is a "sure, fair enough" kind of neutral acknowledgment of a statement or opinion. Like you said, it's more ok/ I get your point. I don't know if I would go as far to say it is "approving," it definitely is more neutral for me. I've never used it in any instances like you've mentioned in number one.

Perhaps as an expression of exasperation when you haave given up trying to reason with someone. e.g. You want to go over Victoria Falls in a barrel? Fair enough, it's your life! Most commonly at the end of a statement of intent. Fair enough?(Do you understand? / Am I being reasonable?)