Salt is very important to us now, but it might be easy to forget that it may not have always been so cheap and available. Does salt have a dark history the way, let's say, some spices do?
The value of salt during different periods of time has been discussed at some length by u/wcspaz, u/AncientHistory, u/Lunx_Rufus, u/wotan_weevil, and others. See this thread.
There is a mention of threats of salt-deficiency being used to coerce subjects which touches on your final subquestion and perhaps one of these authors or another will provide a more in-depth answer with respect to that aspect of salt's history in particular but there is plenty of material there to get you started.
Pleasant reading!