|The Lord said to Moses: "Come to Pharaoh, for I have hardened his heart and the heart of his servants, in order that I may place these signs of Mine in his midst,||א. וַיֹּאמֶר ה אֶל משֶׁה בֹּא אֶל פַּרְעֹה כִּי אֲנִי הִכְבַּדְתִּי אֶת לִבּוֹ וְאֶת לֵב עֲבָדָיו לְמַעַן שִׁתִי אֹתֹתַי אֵלֶּה בְּקִרְבּוֹ:|
My intuition tells me that this is because Pharoah tries bargaining with Moshe, in terms of who may leave Egypt for the chag, and with what. If Moshe's priority is to get the Hebrews out of Egypt, then he may well give in. However, with Hashem's directive here, the priority changes. If Pharaoh gives in entirely, fine. If not, it is further hardening of the heart, and the bringing of the makkah upon failed negotiation is indeed a good thing.
2) The next pasuk:
|and in order that you tell into the ears of your son and your son's son how I made a mockery of the Egyptians, and [that you tell of] My signs that I placed in them, and you will know that I am the Lord."||ב. וּלְמַעַן תְּסַפֵּר בְּאָזְנֵי בִנְךָ וּבֶן בִּנְךָ אֵת אֲשֶׁר הִתְעַלַּלְתִּי בְּמִצְרַיִם וְאֶת אֹתֹתַי אֲשֶׁר שַׂמְתִּי בָם וִידַעְתֶּם כִּי אֲנִי יְהֹוָה:|
I don't know that we explicitly darshen this. However, this seems like a source for listing the makkos, and Detzach Adash BeAchav, on the seder night. As well as other otot. It is not just darshening from Arami Oved Avi, but a specific listing of the signs.
3) In perek 11:
|and every firstborn in the land of Egypt will die, from the firstborn of Pharaoh who sits on his throne to the firstborn of the slave woman who is behind the millstones, and every firstborn animal.||ה. וּמֵת כָּל בְּכוֹר בְּאֶרֶץ מִצְרַיִם מִבְּכוֹר פַּרְעֹה הַיּשֵׁב עַל כִּסְאוֹ עַד בְּכוֹר הַשִּׁפְחָה אֲשֶׁר אַחַר הָרֵחָיִם וְכֹל בְּכוֹר בְּהֵמָה:|
|to the firstborn of the captive: Why were the captives smitten? So that they would not say, “Our deity has demanded [vengeance] for their [our] degradation, and brought retribution upon Egypt.” -[from Mechilta, Bo, on Exod. 12:29]||עד בכור השבי: (שמות יב כט) למה לקו השבויים, כדי שלא יאמרו, יראתם תבעה עלבונם והביאה פורענות על מצרים:|
|from the firstborn of Pharaoh… to the firstborn of the slave woman: All those inferior to the Pharaoh’s firstborn and superior to the slave woman’s firstborn were included. Why were the sons of the slave women smitten? Because they too were enslaving them [the Israelites] and were happy about their misfortune. — [from Pesikta Rabbathi, ch. 17]||מבכור פרעה עד בכור השפחה: כל הפחותים מבכור פרעה וחשובים מבכור השפחה היו בכלל. ולמה לקו בני השפחות, שאף הם היו משעבדים בהם ושמחים בצרתם:|
|and every firstborn animal: Because they [the Egyptians] worshipped it, and when the Holy One, blessed be He, punishes any nation, He punishes its deity. — [from Mechilta, Bo, on Exod. 12:29]||וכל בכור בהמה: לפי שהיו עובדין לה, וכשהקדוש ברוך הוא נפרע מן האומה נפרע מאלהיה:|
Why bring in the captive, who are listed in place of the slave, in the actual fulfillment of the plague? Because here is the place to discuss why all these seeming innocents are being smitten.
We didn't (necessarily) ask before, by the previous plague attacking cattle, what the poor cows did. Nor did we ask what the poor crops did. Aside from this midrashic answer, I think the answer is that these are all tafel. What did the firstborn babies do? Or were they punished for their parents' actions? Rather, they are tafel. The purpose is an all-encompassing punishment for all Egypt, and a distinction from the Hebrews, who are entirely spared from this punishment. As it states two pesukim later:
|But to all the children of Israel, not one dog will whet its tongue against either man or beast, in order that you shall know that the Lord will separate between the Egyptians and between Israel.'||ז. וּלְכֹל | בְּנֵי יִשְׂרָאֵל לֹא יֶחֱרַץ כֶּלֶב לְשֹׁנוֹ לְמֵאִישׁ וְעַד בְּהֵמָה לְמַעַן תֵּדְעוּן אֲשֶׁר יַפְלֶה ה בֵּין מִצְרַיִם וּבֵין יִשְׂרָאֵל:|
If any distinctions were made within the Egyptians, then this point would not be made. Indeed, that is possibly the point of the Mechilta on Bo which Rashi cited above in עד בכור השבי.