Why did they not tell their father that Shimon was incarcerated, and that they too had been incarcerated for three days?
Malbim: They said that Yosef merely told him Shimon to stay, to encourage Yaakov to fulfill their request of sending Binyamin down to Egypt with them.
Ha'amek Davar says that this is why they did not tell him that Yosef wanted to incarcerate all of them. Perhaps he means that Yosef wanted to incarcerate all of them, until he would verify their claim (refer to 42:20:152:1), or he would let only one go to get Binyamin, and nine (a strong majority) is considered all of them. (PF)
They saw how much he suffered when he was informed about Yosef. (EC)
Why did they say "Achichem ha'Echad"? Yosef said "Achichem Echad"!
Ha'amek Davar: Yosef intended that they themselves will pick one of them. At the end, Yosef himself picked Shimon; it is as if from the beginning, he said that he will pick one (ha'Echad).
How did they explain how they will verify their claim?
Malbim: If you are not brothers, no one would be willing to be security until the others return, lest the others not return and he will be Chayav Misah! This does not prove that they are not spies. For this, we must bring our small brother. A father would not send their small brother to danger (if they really are spies; perhaps the truth will be revealed)!
