Some of these interpretations are mine:
The blessings of course are prophecies. They do not actually come from Yitzchak's own words but from G-d.
In order to "find" a blessing for Eisav after the original one had been "taken" by Yaakov, Yitzchak had to see Ruach Hakodesh.
The G'mora identifies the prophets and all the Patriarchs, including Yitzchak, are included among them. Where else do we find Yitzchak pronounce any prophecies? So clearly they must be here.
Yitzchak tells Eisav that Yaakov was made master over him then asks him "what can I give to you". The response is here:
- And Esau said to his father, "Have you [but] one blessing, my father? Bless me too, my father." And Esau raised his voice and wept. לח. וַיֹּאמֶר עֵשָׂו אֶל אָבִיו הַבֲרָכָה אַחַת הִוא לְךָ אָבִי בָּרֲכֵנִי גַם אָנִי אָבִי וַיִּשָּׂא עֵשָׂו קֹלוֹ וַיֵּבְךְּ
Rashi comments on the first part (have you but one blessing) but not on the weep.
Have you [but] one blessing: The“hey” [in הַבִרָכָה] indicates an interrogative expression, as in (Num. 13:19):“are they in open cities (הַבְּמַחֲנַיִם) ?” ;“is it fat (הַשְּׁמֵנָה) ?” ; (II Sam. 3:33):“[Should Abner die] like the death of (הַכְּמוֹת) a wicked man?”
הברכה אחת: ה"א זו משמשת לשון תמיה, כמו (במדבר יג יט) הבמחנים, (במדבר יג כ) השמנה היא, (ש"ב ג לג) הכמות נבל: Earlier he cried a "bitter cry" and that didn't yield any real sympathy, but what is this weep?
Perhaps that was, for one moment, a form of Teshuvah which warranted him being given the blessing. (I'll have to find my Rabbinic commentary sources about the great cry). Unfortunately, as we will soon see, it didn't last. Not for him personally, anyway.
Yitzchak might have answered to Eisav the same way that Yaakov does to Rachel when she asks for children. Something in the nature of "Blessings come from G-d". But Yitzchak knew Eisav and we can probably guess why he didn't bother saying it.
In any case, G-d is now about to give over the words for the role He has in mind for Eisav. And his descendents. So even if Eisav is "wicked" and will not listen, he will have some righteous descendents (he did). And of course, like everybody else, he had free-will.
- And his father Isaac answered and said to him, "Behold, your dwelling place shall be the fat places of the earth and of the dew of the heaven from above. לט. וַיַּעַן יִצְחָק אָבִיו וַיֹּאמֶר אֵלָיו הִנֵּה מִשְׁמַנֵּי הָאָרֶץ יִהְיֶה מוֹשָׁבֶךָ וּמִטַּל הַשָּׁמַיִם מֵעָל:
- And you shall live by your sword, and you shall serve your brother, and it will be, when you grieve, that you will break his yoke off your neck." מ. וְעַל חַרְבְּךָ תִחְיֶה וְאֶת אָחִיךָ תַּעֲבֹד וְהָיָה כַּאֲשֶׁר תָּרִיד וּפָרַקְתָּ עֻלּוֹ מֵעַל צַוָּארֶךָ:
From the first verse and half of the second, the role of supporting his brother seems to be clearly the role there. He will fight the battles for him (living by the sword) and will have wealth and power, but should use it to support (serve) his brother Yaakov.
Rashi and others interpret the second part of v40 (when you grieve..) to mean that it applies only when Israel is righteous.
and it will be, when you grieve: [תָּרִיד] is an expression of pain, as in (Ps. 55:3):“I will lament (אָרִיד) in my speech” ; i.e., when the Israelites will transgress the Torah, and you will have cause to grieve about the blessings that he took, “you will break his yoke,” etc. [From Targum Onkelos]
והיה כאשר תריד: לשון צער, כמו (תהלים נה ג) אריד בשיחי, כלומר כשיעברו ישראל את התורה, ויהיה לך פתחון פה להצטער על הברכות שנטל, ופרקת עלו וגו
As you say, Eisav didn't want this role. He wanted the power for himself. As is clear from the next verse...
- And Esau hated Jacob because of the blessing that his father had blessed him, and Esau said to himself, "Let the days of mourning for my father draw near, I will then kill my brother Jacob. " מא. וַיִּשְׂטֹם עֵשָׂו אֶת יַעֲקֹב עַל הַבְּרָכָה אֲשֶׁר בֵּרֲכוֹ אָבִיו וַיֹּאמֶר עֵשָׂו בְּלִבּוֹ יִקְרְבוּ יְמֵי אֵבֶל אָבִי וְאַהַרְגָה אֶת יַעֲקֹב אָחִי:
Given the curses given to the descendents of some people who did "evil" (male descendents of Moav and Ammon, for example) Eisav didn't do too badly.
His descendents can convert and, from the 3rd generation, marry into the nation. So righteous individual descendents can get the "convenant" after all. The prophet Ovadiah, was one such individual.
Israel are told not to "harass" Edom.
Why? Possibly Yitzchak's prayers? (G'morah Megillah states that Yitzchak prayed for the merit of Eisav, although his request was rejected, but we don't know when this happened).
Similar to the way Avraham requested that Ishmael would be elevated before Hashem.