What was the gist of Hashem's answer?
Rashi #2: It was not an answer as much it was a rebuke. Because Moshe queried Hashem's actions, 1 he would see what Hashem was about do to Pharaoh, but not what He would later do to the seven nations. 2
Rashi #1 (in Sanhedrin 111a): Having complained about Pharaoh's bitter decree, now you will see the miracles that I am about to perform on Pharaoh.
Ramban: Hashem assured him that, not only would it not get worse, but that he was about to witness how Hashem would turn the table on Pharaoh, with immediate effect, and that his days were about to come to an end.
Seforno: Moshe was now about to witness how, with the same exertion that he applied to hold on to Yisrael and to scatter them throughout the land, Pharaoh would exert himself to expel them from the land - every last one of them. 3
With whose strong hand would Pharaoh send Yisrael out?
Rashi, Targum Onkelos and Targum Yonasan: He would send them out on account of Hashem's strong Hand (that would eventually force him to capitulate).
Rashbam: He would expel Yisrael with a strong hand, to force them to leave against their will. 1
And with whose strong hand would Pharaoh expel Yisrael from his land?
Rashi: He would expel Yisrael with a strong hand, to force them to leave against their will, so that they did not have time to prepare food for the journey 1
Targum Onkelos and Targum Yonasan: He would expel them too, on account of Hashem's strong Hand.
As the Torah writes in Bo (12:33 [Rashi]).
Rashi writes that Avraham did not question Hashem. Avraham said "ba'Meh Eda"!
Da'as Zekenim, Hadar Zekenim, Riva: Avraham did not question Hashem, he merely asked to know the merit in which he will inherit the land. Hashem answered, in the merit of Korbanos.
Rashi writes that because Moshe questioned Hashem, he would not see what He will do to the seven nations. Below, "Ya'an Lo He'emantem Bi", connotes that it was due only to Mei Merivah!
Riva: Questioning Hashem caused that Moshe not see vengeance against the seven nations. Due to Mei Merivah, he could not enter Eretz Yisrael.
Why are there multiple expressions "Yeshalchem... Yegarshem", Garesh Yegaresh (11:1)?
Rosh: Midas ha'Din argued that 400 years of servitude were decreed. In order to reduce them, it was necessary that Pharaoh explicitly free them (the repetitions teach this).