What is the significance of the fact that the Torah places Yitzchak before Yishmael?
Rashi: This is to teach us that Yishmael did Teshuvah 1 and acknowledged Yitzchak's precedence 2 over himself at the Levayah of their father.
Ramban (citing the Midrash): We see here how the son of the maidservant gave Kavod to the son of the main wife.
Why did the Bnei Keturah not participate in the burial of their father?
Malbim (in 25:6): Because Avraham had sent them far away (and presumably, they did not even hear that he died - Oznayim la'Torah).
Why does it say "El Me'aras," and not b'Me'aras?
Refer to 23:19:154:1 and the note there.
QUESTIONS ON RASHI
Rashi writes: "Yitzchak and Yishmael' - This proves that Yishmael did Teshuvah, and placed Yitzchak ahead of himself." But perhaps Yishmael did not change, and the Torah lists Yitzchak first only on account of his wisdom and stature?
Gur Aryeh: Later, at Yitzchak's burial, the Torah lists Eisav before Ya?akov (35:29), despite the fact that Ya?kov was greater. This proves that Yishmael did Teshuvah and allowed Yitzchak to precede him - whilst Eisav did not.
Rashi writes: "This shows that Yishmael did Teshuvah, and placed Yitzchak ahead of himself." Perhaps Yishmael did so only because he was the son of the maidservant, while Yitzchak was the son of the mistress of the house - but it does not indicate any Teshuvah on his part?
Gur Aryeh: If Yishmael was still a Rasha, he would hate the Tzadik, Yitzchak; and that hatred would blind him even to the rules of propriety. The fact that Yishmael no longer hated Yitzchak demonstrates that he had done Teshuvah, and he allowed Yitzchak to go first, as was fitting.
