1)
Verse 25 said "Shemor l'Avdecha." Why does it say here "Ye'amen Devarcha l'Avdecha"?
1.
Malbim (25-26): Indeed, the promise of permanent kingship was conditional. I request that you cause its fulfillment, supervising that his descendants not veer from Hashem's ways. It is not proper to nullify the promise to David, which was amidst Chesed, due to sins of his descendants.
2)
Why is it written Devarecha, and we pronounce it Devarcha?
1.
Radak: Devarecha refers to the Prat (each word; there are many), and Devarcha refers to the Klal (what You said; this is singular).
