1)
Why did HaSh-m make the covenant of Milah with Avraham at this juncture?
1.
Ramban (citing Ibn Ezra): Praised be HaSh-m, who ordered Avraham to perform the Bris Milah now, before Sarah became pregnant, so that Yitzchak should be conceived in Kedushah!
2)
Why is "Av Hamon Goyim" written twice (here in Pasuk 17:4, and again in 17:5)?
1.
Ba'al ha'Turim: The Gematriya of "Av Hamon " (104), twice, is equivalent to that of "Yitzchak" (208). 1
1
As well as to the Gematriya of "Arbeh" - "I shall increase" (Bereishis 17:2, 22:17).