Can we infer from "va'Teled Lo" that Halachically, Bnei Keturah are considered Avraham's children?
Seforno: We cannot. It says so because Avraham raised them in his home. (Refer to 25:4:151:1.)
Why does the Torah not stress the miracle that Avraham fathered more children in his old age?
Ha'Emek Davar: Once his ability to have children was opened, it became his nature 1 , just like the rock in the Midbar. Once it gave water, this became its nature.
This was at least 40 years after Yitzchak was born. Yitzchak was born only 14 years after R. Yishmael. Why does the Torah stress (18:11) that Avraham would father Yitzchak in his old age? Yishmael was an aberration; he was born before Hashem changed Avram's name to Avraham. Astrology taught that Avram cannot father! (Heard from R. Moshe Shapiro)