1)
When did David compose this Mizmor?
1.
Radak: It was when he was exiled and fleeing Sha'ul.
2.
Radak citing Shocher Tov: He said it about Keneses Yisrael in exile.
3.
Malbim: His heart expressed a good matter, and said that both when Sha'ul was chasing him on rocks of Ye'elim (wild goats), and when he reached kingship, he requested the same matter - to always sit in Beis Hashem and serve Him. This is like he said, "Achas Sha'alti me'Es Hashem
2)
What is "Neginas"?
1.
Radak: This is not Samuch, even though there is a Patach [before the Sov and not a Cholem]. It is as if it says Neginah, with a Hei. We find like this, that it is not Samuch - "Makas Bilti Sarah" (Yeshayah 14:6).