What role would the officers play?
Ramban and Rashbam: To collect the produce throughout the land (for storing away during the bountiful years), since it would not be possible for the viceroy to do it himself.
Rashbam (to 41:35): The officers were needed to enforce the law, since the people would not be willing to go along with it.
Moshav Zekenim: Yosef said to appoint overseers (plural), so if he would not be the head, at least he would be one of them.
Ohr ha'Chayim: Normally, after a bumper crop, people would be lazy and not seed so much, for they already have extra grain, and it is cheap. After three or four years, they would cease working the land! The dream came to inform that they should grow as much as they can during the seven years, to prepare for the famine.
Malbim (to 41:35): They will save all food. Food other than grain, e.g. legumes and fruits, they will oversee it in cities, and guard it [from theft].
What is the significance of the opening words, "Ya'aseh Pharaoh"?
Bechor Shor, R. Bechayei, Ba'al ha'Turim: Pharaoh will instruct the Chacham to appoint officers. Seforno, Ha'amek Davar - He will appoint in each and every town, so that they should acknowledge the viceroy as their head, and as such, they would be obligated to follow his instructions, and they will fear him.
Alshich: If Pharaoh does not want to appoint one Chacham over Mitzrayim, he should appoint officers.
RS"R Hirsh: Pharaoh 1 will appoint officers.
Perhaps the Targumim, Rashi and the other Mefarshim who did not specify hold like this, for the verse discusses Pharaoh, and did not say that someone else will appoint the officers. Normally, when the subject is omitted, the Mefarshim say so. (PF)
What is the meaning of the word "v'Chimesh"?
Rashi, from Targum Onkelos: It means "and he will make [them] zealous." 1 This is like "va'Chamushim Alu Bnei Yisrael" (Shemos 13:18). 2
Targum Yonasan, Rashbam, Ibn Ezra, Radak, Da'as Zekenim, Hadar Zekenim, Ohr ha'Chayim: It is to take a fifth 3 (Chomesh) of all the crops.
Ha'amek Davar: He will divide Egypt into five districts with an officer over each. He learned from the first dream. Two evil cows ate the seven fat ones; five thin cows did no evil (refer to 41:4:151:1). I.e. five districts will sustain the satiation, so the years of famine will not consume it without benefit. Even though the fact that two cows ate the seven teaches that there will be only two years of famine (before Yaakov's arrival interrupts it), the dream is Hashem's word; it has many explanations.
Ohr ha'Chayim: I.e. they should not be lazy from working the land.
There, Rashi explained that va'Chamushim means armed! Yalkut Shimoni there (227) says that these are two different matters! (PF)
Rashbam: This is twice the king's customary tax of a tenth, see Shmuel I 8:14,15. See also later, Bereishis 47:26. See also Oznayim la'Torah.
Many (refer to 41:34:3:2) explain that v'Chimesh means taking a fifth. The next verse says that all the crops were under the jurisdiction of Pharaoh!
Oznayim la'Torah (to 41:35): Pharaoh took a fifth as a tax, and bought the rest of what remained [from what the people needed to eat now]. He did not leave it in individuals' jurisdiction, lest they inflate the price [in the famine years] and cause a hunger of panic.
