What are the implications of the words "Koh Somar ... "?
Rashi: It implies to say the following words without change. 1
Rashi did not add 'in this exact order' as he did above in Sh'mos 19:3. See Sifsei Chachamim here and there.
What are the ramifications of the Pasuk "Atem Re'isem ... "?
Rashi: It is impossible to deny what one has seen! 1
Ramban: Having seen with their own eyes what they saw and that Hashem is Master over Heaven and earth, Yisrael are prohibited from adopting or making any other gods. 2
Seforno: It serves as a practical lesson that, although Hashem dwells in the Heaven, He lowers Himself to supervise His creations and to communicate with them. 3
As the old saying goes 'Seeing is believing!' Consequently, Yisrael will never be able to deny Ma'amad Har Sinai..
As the Pasuk says in Tehilim, 113:5 & 6.
How will we reconcile this Pasuk with the Pasuk earlier in Sh'mos 19:20 -"Vayeired Hashem al Har Sinai"?
Rashi #1: Hashem's Kavod in the form of His Voice) was in the Heaven - to chastise them, and His fire and great might were on earth. 1
Rashi #2: He bent the Heaven and the Heaven of the Heavens and spread them on the Mountain. 2
Ramban: Hashem was in Heaven and His Kavod (in the form of the fire) was on Har Sinai. 3
Rashi: As the Pasuk indicates in Va'eschanan Devarim, 4:36.
Rashi: As the Pasuk indicates in Tehilim, 18:10 (Rashi). Refer also to 19:20:1:1.
Ramban: As the Pasuk indicates in Nechemyah, 9:13. The Ramban agrees with Rashi's quotations, but disagrees with his interpretation. He also quotes the Ibn Ezra, who cites Ki Sisa Sh'mos, 33:21 (See R. Chaval's notes).