1)
Why does it say "Rabas Shachnah Lah Nafshi"?
1.
Radak: The Sov at the end of Rabas is in place of a Hei; it is not Samuch. The same applies to "Rabas Tzareruni" (129:1), "Rabas Taserenah" (65:10). I dwelled much in Galus with a nation that hates Shalom.
2.
Malbim: This evil is greater, that I dwelled with one who hates Shalom. I dwell among my brothers Bnei Yisrael, who hate Shalom. This is worse than Bnei Kedar and Meshech (refer to 120:7:2:3)