Having just Davened (See Seforno (Pesukim 6 - 9), why did Moshe not wait for a reply before proceeding to issue instructions?
Seforno #1: He instinctively knew that his prayers had been answered, because they came out fluently.
Seforno #2: Because Moshe Rabeinu's Tefilos on behalf of K'lal Yisrael were always answered.
As the Gemara in B'rachos, 34b, says with regard to R. Chanina ben Dosa in B'rachos, 34b (Seforno).
Presumably that is what the Seforno means when, after the previous explanation, he adds 'O Yoser mi'Zeh'.
Where exactly was "Lifnei Hashem"?
Rashi: "Lifnei Hashem" meant in front of the location where the Cloud (of Glory, containing the Shechinah [Targum Yonasan]) was about to appear.
As the Torah specifically writes in Pasuk 10. See also Seforno.