The plot of the play focuses on one subject only: Creon's command not to not bury Polyneices, and Antigone's defiance of that command because he is her brother and the gods demand burial of the dead. There are no subplots. All other characters only serve to enhance the theme and conflict above. Ismene is afraid. Creon's son, Haemon supports his fiance, Antigone. There is no "action" in the play, it all only a series of conversations, and those conversations are thought-provoking, argumentative, and emotional. The audience can see Creon's point about Polyneices seeming to be a traitor, but still knows that Creon is ultimately wrong because a human should never try to defy the gods. Antigone's strength and determination are very admirable, and her threat of suicide is made real. It is added drama that Haemon also commits suicide and that Creon only realizes the error of his thinking in a moment just minutes too short to save the young people from doing themselves in. All of that end of the play is very gripping and emotional, thus providing what Aristotle would have call the catharsis.
Antigone does follow most of the elements needed for a good story according to Aristotle. Of course, they share similar cultural values because they come from the same culture. There is high drama coming from a difficult choice caused by tragedy, thus creating its own tragedy.
Aristotle's classical unities for drama are pretty simple. Basically the action within a play should stay within a limited physical space, should have unity of action (i.e. few subplots, tangents, and deviations from the tragic form), and should occur within a limited amount of time. In Poetics, he gives the most attention to unity of action, and Antigone sticks to the plotline pretty rigorously. Aristotle's unities are essentially guidelines for elegance, and few plays are as elegant as Antigone.
In reply to #2 and #3: how would apply them with these clues
-the plot is in one piece but has a highly dramatic quality.
ps. i want to make a whole paragraph on this topic.