1)
Verse 9 already mentioned that he will descend to She'ol. Why is this repeated?
1.
Radak: You thought to ascend above Kochvei Kel (13). Instead, you will descend to She'ol.
2.
Malbim: You thought to ascend from the depth to Shamayim. Just the contrary, you will descend to She'ol!
2)
What is the significance of "Yarkesei Vor"?
1.
Radak: You thought to dwell in Yarkesei Tzafon (13). Instead, you will descend to Yarkesei Vor, i.e. the grave.
2.
Malbim: You thought to rule over all Kochvei Kel, i.e. ascend from the depth to Shamayim. Just the contrary, your Reshus will be limited to the grave - not one Amah outside of it!