What are the connotations of the word "Leimor" in this Pasuk?
Why did Moshe assume that Par'oh would not listen to him?
Rashi (according to the Sifsei Chachamim): If Yisrael, who were being given good news, did not listen to him, how much more so Par'oh, for whom the news was bad!
Ramban #1: Since Hashem had not engineered that Yisrael should listen to him, it went without saying that Par'oh would not listen to him either, let alone the fact that he had a speech defect and it was not befitting for him to speak before such a great king.
Ramban #2: He thought that Yisrael did not listen to him on account of his speech defect, despite the fact that he gave them comforting news, and if he could not convince them, how would he convince Par'oh, for whom the news was not so good.
Seforno: He thought that Yisrael did not listen because he failed in his first Shelichus. In which case, Par'oh, who got his way, would certainly not listen to him.
Alshich: If B'nei Yisrael, who are believers the sons of believers, and who know that Moshe was the redeemer, did not believe him
Before whom only eloquent speakers would generally speak (PF).
Hashem already informed Moshe that Aharon would speak on his behalf, so why did Moshe again reiterate his speech defect?
Ramban: The first time Hashem instructed him to speak to the people, Moshe complained that he was not a man of words - See Sh'mos, 4:10, to which Hashem responded by appointing Aharon as his interpreter (Ibid. 4:16). However this time, Hashem ordered him to speak personally to the people (above, Pasuk 6), but without success. Consequently, when Hashem now instructed him (in Pasuk 10) to speak to Par'oh, he complained once more about his problem. This time too, Hashem responded (in the next Pasuk) by sending Aharon together with him to speak on his behalf.
Seforno: Because he thought that Aharon only spoke on his behalf on the previous occasion when he spoke to the B'nei Yisrael, but that when he spoke to Par'oh, he would have to do so himself.
Why does the Torah say "Lifnei Hashem" and not "el Hashem"?
Oznayim la'Torah: To teach us that he was simly making a point
How could Moshe make a Kal va'Chomer that Par'oh would not listen to him, when the Torah attributes Yisrael's non-compliance to shortness of breath and hard work!
Mizrachi: Moshe did not know that this was why B'nei Yisrael did not listen to him.
Alshich: Indeed that is precisely why Moshe added that he had a speech defect.
Da'as Zekenim and Hadar Zekenim: Kotzer Ru'ach was due to the hard work