Whom did they appoint as a Shali'ach to approach Yosef on their behalf?
Rashi: They appointed the sons of Bilhah (i.e. Dan and Naftali) as their Sheluchim. 1 Yosef was friendly with them since the outset (37:2, see Rashi). 2
Seforno: They commanded Yaakov's servants or somebody else to go and speak to Yosef.
Rashbam: They commanded modest emissaries to speak to Yosef before they went in to speak to him.
Targum Yonasan: It was Bilhah (herself) whom they sent to speak to Yosef.
Nowhere do we find that Yaakov actually issued such a command!
Rashi: He didn't - because he did not suspect Yosef of planning revenge. It was the brothers who concocted it, with a view of making peace with Yosef.
Seforno: Although the Torah does not record it, Yaakov (who did not himself suspect Yosef of such bad Midos) ordered the brothers to tell Yosef that he had ordered them to say the following, should they be afraid that he is planning revenge.
Why did Yosef weep as they spoke with him?
Seforno: He wept when they mentioned his father and the love he bore him, in that he did not suspect him of taking revenge.
Why does it say "va'Yetzavu El Yosef Leimor"?
Ohr ha'Chayim: If it means to say to Yosef, it should say "Leimor El Yosef"! Perhaps they intended that the messenger will not say that the brothers sent him, rather, that he heard their father command to the brothers to say these matters to him. This is why it says "Leimor" twice.
How could Yosef believe that Yaakov commanded so before his death? Yosef was with him when he died!
Ha'amek Davar: They made Yosef understand that in Yaakov's Berachah, "va'Yafozu Zero'ei Yadav" (49:24) - he praised him for not shooting arrows and a bow against those who embittered him. Through this, he commanded to tell that this is his desire.
Why did the brothers wait until after Yaakov died before appeasing Yosef?
Da'as Zekenim: Initially, they thought that Yosef had forgotten the matter. There was no reason to arouse the hatred. When they returned from burying Yaakov, they saw that he had not forgotten. Refer to 50:15:1:2.