Why was Moshe unable to keep his hands aloft?
Rashi #1: It was a punishment for being lax in the Mitzvah - by sending Yehoshua to war to lead the troops and not going himself. 1
Rashi #2 (citing B'rachos, 54b) and Ramban #1: Because he was physically unable to do so without support.
Ramban #2(citing the Midrash): Because it is forbidden to stand with hands raised for three hours.
Targum Yonasan: It was a punishment for delaying the battle for the next day and not engaging in the defense of Yisrael immediately. 2
Why did they take a stone for Moshe to sit on and not a cushion or something softer?
Rashi and Targum Yonasan: Since Yisrael were suffering, Moshe made a point of suffering together with them.
What are the connotations of the word "Emunah" in this Pasuk?
Rashi and Targum Yonasan: It refers to Moshe's Tefilah, which he prayed with deep Emunah, hands outstretched.
Ramban #1: It means that his hands did not sag but remained firmly aloft without letting up until sunset. 1
Malchus, Y'sod, Hod, Netzach, Tiferes, Gevurah, Chesed, Chochmah, Binah, Da'as.
This is the same concept as the Kohanim, who stretch out their hands when they Duchen (Ramban). See also Ba'al ha'Turim, who explains that "Emunah" refers to the merit of Avraham, "Ad Bo", to the merit of Yitzchak and "ha'Shemesh", to the merit of Ya'akov.
What is the significance of sunset here?