1)
The verse implies that Avraham fathered only Yitzchak!
1.
Malbim: It says "Ki v'Yitzchak Yikarei Lecha Zara."
2)
Why is he called Yisrael, and not Yakov?
1.
'Rashi': This is for the honor of David 1 .
1
Yisrael is a higher level than Yakov (Alshich Bereishis 47). Alternatively, Yakov implies that he received the Berachos via Akvah and trickery; Yisrael implies openly, with authority (Rashi Bereishis 32:29). (PF)
