Beshalach: Why Did Yosef Adjure The Descendants?

What is bothering Rashi here, that he resorts to a midrash? How is this midrash an aggada hameyeshevet divrei hamikra?

The very beginning of parshat Beshalach, we encounter:

19. Moses took Joseph's bones with him, for he [Joseph] had adjured the sons of Israel, saying, God will surely remember you, and you shall bring up my bones from here with you.

for he had adjured Heb. הִֹשְבִּיעַ הַֹשְבֵּעַ. [The double expression indicates that] he [Joseph] had made them [his brothers] swear that they would make their children swear (Mechilta). Now why did he not make his sons swear to carry him to the land of Canaan immediately [when he died], as Jacob had made [him] swear? Joseph said, “I was a ruler in Egypt, and I had the ability to do [this]. As for my sons-the Egyptians will not let them do [it].” Therefore, he made them swear that when they would be redeemed and would leave there [Egypt], they would carry him [out]. — [from Mechilta]
and you shall bring up my bones from here with you He made his brothers swear in this manner. We learn [from this] that the bones of all [the progenitors of] the tribes they brought up [out of Egypt] with them as it is said “with you” -[from Mechilta]

What compels Rashi to bring up this derasha of the duplicate language of הַשְׁבֵּעַ הִשְׁבִּיעַ. After all, on the level of peshat, he can simply say that dibra Torah kilshon benei Adam. But this is close reading that he adjured them to hishbiah, themselves cause others to be sworn to do. What compels this?

Furthermore, note that there is ambiguity here, in that בְּנֵי יִשְׂרָאֵל can refer either to the nation of Israel or to the sons of Yaakov. In both explanations, he explains this as referring to the sons of Yaakov, rather than to the descendants. This ties in with the making of Yosef's brothers adjure their own children. And thus "with you" would refer to the bones of Yosef's brothers, to whom he is speaking.

The issue is perhaps one of chronology, with some basis on the fact that Yosef was not the last brother to die -- Levi was. If so, why not reenact the burial that Yaakov had.

But it is more than that. If we look at the first perek of Shemot:

ו וַיָּמָת יוֹסֵף וְכָל-אֶחָיו, וְכֹל הַדּוֹר הַהוּא.	6 And Joseph died, and all his brethren, and all that generation.
ז וּבְנֵי יִשְׂרָאֵל, פָּרוּ וַיִּשְׁרְצוּ וַיִּרְבּוּ וַיַּעַצְמוּ--בִּמְאֹד מְאֹד; וַתִּמָּלֵא הָאָרֶץ, אֹתָם. {פ}	7 And the children of Israel were fruitful, and increased abundantly, and multiplied, and waxed exceeding mighty; and the land was filled with them. {P}
ח וַיָּקָם מֶלֶךְ-חָדָשׁ, עַל-מִצְרָיִם, אֲשֶׁר לֹא-יָדַע, אֶת-יוֹסֵף.	8 Now there arose a new king over Egypt, who knew not Joseph.
ט וַיֹּאמֶר, אֶל-עַמּוֹ: הִנֵּה, עַם בְּנֵי יִשְׂרָאֵל--רַב וְעָצוּם, מִמֶּנּוּ.	9 And he said unto his people: 'Behold, the people of the children of Israel are too many and too mighty for us;
י הָבָה נִתְחַכְּמָה, לוֹ: פֶּן-יִרְבֶּה, וְהָיָה כִּי-תִקְרֶאנָה מִלְחָמָה וְנוֹסַף גַּם-הוּא עַל-שֹׂנְאֵינוּ, וְנִלְחַם-בָּנוּ, וְעָלָה מִן-הָאָרֶץ.	10 come, let us deal wisely with them, lest they multiply, and it come to pass, that, when there befalleth us any war, they also join themselves unto our enemies, and fight against us, and get them up out of the land.'

it really seems that the servitude did not begin until after the death of Yosef, when a Pharaoh arose who did not know Yoseph. If so, what would Yosef mean by "God will surely remember you?" The servitude did not yet start! Thus there was some predictive element, that Pharaoh would not let his sons or brothers take his bones out, based on the present attitude of Pharaoh. Yet he knew (on a peshat level) the prophecy to Avraham about the prolonged servitude and ultimate redemption. He would have to be talking to those in his own generation -- his brothers, the bnei yisrael; yet the oath was for future generations (the benei Yisrael) to fulfill, as he himself says. And this difficulty and resolution is brought to the fore by the midrash in the Mechilta.