Izanami is still a genjutsu that traps you in it for as long as possible till you accept your actions or figure out you're in a loop of genjutsu . Even if you only stay in Izanami for 1 or 2 seconds , Itachi could walk up to you and cut your head off.
He doesn't know how to break it , so to him its a loop of actions he and Itachi shared which were recorded with Itachi's sharingan so once Itachi plays Izanami for Minato, Minato will be reliving it over from point A to point C and back to point A again. Before he makes it back to point A , Itachi can cut Minato's head off. Izanami is a real genjutsu and Minato's body will be sitting in one spot while his mind is in a genjutsu. Itachi isn't in the genjutsu so he can chop off Minato's head. Before Minato even watches the complete loop 1 time, Itachi can kill him by then.
anyone can fall into the izanami genjutsu if only for a few seconds, the victim would have to figure out the loop and when they are in the izanami they will relive point A to point B & C and back to point A again over and over.
While you are in Izanami if only for at least 1 second, Itachi (the izanami user) can chop your head off in the real world.
Your head getting chopped off results in your death. After that, you're no longer in izanami, you're dead.

yes Sasuke can use Izanami now after learning it from Itachi by him explaining everything about it to him.