One may suggest that the answers to these questions may be found the nature of the overall scheme which Yosef set out for his brothers. In order to turn their hearts to teshuva, he placed them in a condition which was akin to that of their youth, at which time they resented Yosef and sold him. Thus, Yosef displayed preferential treatment towards Binyomin and placed him in apparent peril so as to test his brothers and determine if they would show love to this younger, favored brother and take responsibility for him. So too, Yosef knew that Binyomin's fate would affect that of Yaakov Avinu. The brothers were thereby faced with a repeated opportunity to relieve their father's anxiety and concern or to augment his emotional suffering. These trials were designed by Yosef to lead his brothers to the path of teshuva, by enabling them to make amends for identical transgressions of years past.
In the same vein, Yosef perceived that by resenting his dreams and attempting to frustrate their prophetic fulfillment, his brothers were trying to force the hand of divine providence. In order to test his brothers in this area, Yosef attempted to impress upon them that their lives and deeds could be omnisciously known and that their fate could be determined and sealed via the goblet and sorcery associated with it. The brothers were challenged with countering and denying the validity of this seemingly all-knowing and all-controlling force and asserting their belief in God's exclusive omniscience and omnipotence. Yehudah's immediate reply to the apparent theft of the goblet, "...God has found the guilt of your servants...", constituted an affirmation of Hashem's providence, signaling to the brothers that the experience was a punishment for their past deeds, in conformity with Yosef's scheme.
May the lessons of teshuvah in Parshas Mikeitz be internalized by us all.