1)
From where were Moshe and the elders 'gathered'?
1.
Rashi: The Pasuk means that they were gathered to their houses (they went home) 1 from the entrance of the Ohel Mo'ed.
1
Rashi: As in Ki Seitzei Devarim, 22:2 and in Tehilim, 39:7.
2)
Why does the Torah need to mention that Moshe and the Zekenim went home?
1.
Rashi: To teach us that Hashem only sent the punishment after all the Tzadikim had gone home. 1
1
As the Pasuk writes in Yeshayah, 57:1 "Ki Mipnei ha'Ra'ah Ne'esaf ha'Tzadik". See Oznayim la'Torah