1)
Why does it say both "ha'Ovrim Es Brisi" and "Asher Lo Hekimu Es Divrei ha'Bris"?
1.
Malbim: They transgressed two Brisim - the Bris of the Torah from before, and the Bris they made now (of Tzidkiyah).
2)
What calf was cut into two?
1.
Rashi: When they returned and seized slaves, they all made a Bris to rebel against Hashem. They cut a calf into two parts, and passed between them. It was a strong absolute Bris, saying that so will be cut and divided one who transgresses it.
2.
Radak: This was the custom of people who made a Bris. The same applies to Bris Bein ha'Besarim of Avraham.
