Why did he insert the Berachah of Avraham and of Eretz Yisrael here (but not in the Berachos that he thought he was blessing Esav)?
Rashi: He was referring to the Berachah of "v'E'eschah l'Goy Gadol" and "v'Hisbarechu b'Zar'acha Kol Goyei ha'Aretz," that that nation and those blessed descendents will come from him. 1
Seforno (to 27:29): This refers to the Berachah of Eretz Yisrael, which was reserved for Yaakov exclusively. 2 Malbim - the Berachah of inheriting Eretz Yisrael is also mentioned with seed, on which HaSh-m will put His Shechinah, for these depend on each other. He attributed this Berachah to HaSh-m, for it is not in the hands of man, as Malbim explained above (refer to 27:1:153:2).
Rashbam: If he will not marry a woman from the Kena'aniyos 3 (who are slaves), he will merit to inherit the land in which he is currently sojourning.
Rashi seems to concur with the Seforno (Refer to 28:4:1:2) in that he never intended to give these particular Berachos to Esav (See however Sifsei Chachamim).
Refer to 27:29:2:1. The Seforno adds here however, that the Berachah of "Birkas Avraham" - "v'Heyei Berachah" which is synonymous with teaching the people the knowledge of HaSh-m - (together with his children) will create the Kidush HaSh-m that is necessary to inherit Eretz Yisrael). The Oznayim la'Torah adds that Yitzchak reserved the Berachah of Avraham for Yaakov, 'the Ish Tam Yoshev Ohalim' who is worthy of it and of inheriting Eretz Yisrael.
Just as Avraham had instructed Yitzchak.
QUESTIONS ON RASHI
Rashi writes: "'Avraham's blessing' - [i.e.] that HaSh-m had said to him, 'I shall make you a great nation' (12:2); and '[the nations] shall bless themselves by your progeny' (22:18)." How does Rashi know that the reference is to these blessings in particular?
Gur Aryeh: Other blessings applied to Avraham personally (e.g. "I shall bless those who bless you" - 12:3); how is Yitzchak able to give them over? The two blessings Rashi cites refer to Avraham's offspring; and Yitzchak blessed Yaakov that he would become that great nation.