The stasis of Medea’s world before Euripides’ play is indeed precarious. Having fallen in love with Jason, Medea helps him to win the Golden Fleece against her father’s wishes, then kills her brother. She leaves Colchis and, in turn, loses everything she has ever known. Jason and Medea sail to Corinth, where she is essentially looked upon as a lower-class barbarian. This is compounded with her nature as an argumentative, vengeful woman with knowledge of magic. Living in an alien land where she is seen as a savage witch, her husband is her greatest support. In Medea’s time and place, that is a dangerous position for a woman indeed.
The inciting incident which tips all of these factors out of balance is Creon’s offering of Glauce to Jason. At first glance, it seems that it could be Jason’s actual announcement that he is leaving Medea. However, it is obvious that Jason highly prizes his social and political position. There is no way that he would refuse Creon’s offer of a young, beautiful bride who can bring him prestige. On the other hand, Jason is very aware of Medea’s nature and her magical abilities. He knows that she will react violently. Therefore, there is no other outcome that could occur from Creon’s offer.
I totally understand the logic behind your choice of inciting incident. However, Jason still has a choice. He doesn’t have to leave Medea for Creon’s daughter. Furthermore, it the text seems to imply that Jason leaves Medea first, and then comes up with the excuse that it’s for her and their children. I think Jason makes some very poor decisions (not a good thing to do when your current wife is a hotheaded witch). It is his first bad move, to agree to marry Glaunce, that sets the play in motion.