Why was Yosef so upset that Yaakov had switched his sons?
Ramban #1: Because he was particularly fond of Menasheh, his Bechor, and he was upset with Yaakov for giving precedence to Efrayim.
Ramban #2: He was worried that his father had erred (confusing Menasheh and Efrayim), and, since he blesses them with the wrong Kavanah, 1 his Berachah would be given without Ru'ach ha'Kodesh. 2
Da'as Zekenim, Hadar Zekenim: How does my father consider me like a fool, that I did not arrange them properly? I arranged them properly!
Consequently, as soon as his father explained that he knew exactly who was who, he calmed down. (Yaakov was blessed when Yitzchak was unaware, and Yitzchak saw no need to repeat those Berachos knowingly! Perhaps there is different, for there was a Nevu'ah that Yaakov get those Berachos, or Yitzchak smelled Gan Eden (refer to 27:27:1:1), and knew that the one in front of him should be blessed, whoever it is - PF.)
Ha'amek Davar: If this was the concern, how will it help to switch his hands? Perhaps Ramban means that Yosef wanted to test whether or not he blesses with Ru'ach ha'Kodesh, or errs, like Yitzchak erred. Yosef was pained that he caused his father to err via putting Menasheh to Yaakov's right (and Yaakov assumed that he was on Yosef's right), and thereby weakened his Bechor's Berachah.
How could Yosef show his father a lack of respect by telling him directly that he was making a mistake?
Rashbam, Riva, Hadar Zekenim (to 48:18): He didn't! 1 He was informing his father that he had not arranged the boys incorrectly, as Yaakov thought 2 - rather, he had taken care to place them correctly.
Ohr ha'Chayim (to 48:18): He was unsure if (a) Yaakov assumed that Yosef put Menasheh on his own right. If so, he informs him that it is not so. (b) Perhaps Yaakov knows that Menasheh is on his own right. If so, he asks why he withholds his right hand from him. You know what this can cause! 3
Rashbam: Had he come to correct his father, he would have asked him why he changed them. He would not have confronted him directly.
Riva: It is normal to place the more important one on one's right. Yosef reversed them, so the Bechor would be on Yaakov's right.
Ohr Yakar: I.e., favoring one son over the others is what led to Galus Mitzrayim!
What is the meaning of "va'Yismoch"?
Ha'amek Davar: He supported the hand of his weak father. He was not so bold to lift it off Efrayim's head; he thought to push it slowly. When this did not work, he voiced his concern (in 48:18).
Did Yosef seek to correct Yaakov before the Berachah, or after?
Ha'amek Davar: It was before. After the Berachah, what was, was! 1
Could Yaakov not give another Berachah, with his right hand on Menasheh?! (PF)
QUESTIONS ON RASHI
Rashi writes: "He supported his father's hand [to remove it...] - [I.e.] he lifted it off his son's head, and supported it." What is Rashi adding?
Gur Aryeh: Merely supporting his father's hand would not move it from its place. Yosef first lifted his father's hand, then supported it [in the air], such that it was on neither son's head.
