If Esav's third wife was called Bosmas Bas Yishmael, why does it refer to her above (28:9) as "Machalas Bas Yishmael"?
Rashi: It is from here that the Midrash learns the principle that when a person gets married, all his sins are forgiven. 1
Ramban (citing Bereishis Rabah 67:13) #1: Bosmas too, was not her real name but a description of Esav's 'fragrant' intent (he wanted to convert), in which case we do not know what her real name was.
Ramban #2 (citing the Ibn Ezra): Both she and her father had two names. 2
Ramban #3: His first two wives (Yehudis Bas Be'eri and Bosmas Bas Eilon) died without children - presumably for having been a thorn in the side of Yitzchak and Rivkah. So he married Adah Bas Eilon (the sister of Bosmas Bas Eilon) and Aholivamah Bas Anah. 3 And he also changed the name of his wife Machalas Bas Yishmael (which has connotations of illness) to Bosmas, the fine name (with connotations of spices) of his first deceased wife.
Rashbam: Refer to 36:3:1:4*. And he married Aholivamah later, after he moved to Har Se'ir, as is evident from 36:25.
Why does the Torah see fit to add that Bosmas was the sister of Nevayos?
Rashi and Targum Yonasan: Because after the death of Yishmael, Nevayos married her off in his place. 1
Which in turn, is important to know because it teaches us Yaakov's age at the time (See Rashi to Bereishis 28:9).
QUESTIONS ON RASHI
Rashi writes: "Three people whose sins are forgiven -- a convert, one who rises to a prominent position, and one who marries." Why is this so?
Gur Aryeh: Each of these three becomes a new, more complete entity; therefore Hashem forgives their previous sins [for they are no longer the same person]. 1
Maharal (Chidushei Agados Vol. 3, p. 136, to Sanhedrin 14a): Sin comes from physicality (Chomer); ascending to a prominent position means ascending above Chomer, that is what brings the atonement. A convert likewise ascends to the higher level of Yisrael. One who marries rises to the level of Chibur (refer to 4:1:1.2). 2
Gur Aryeh: An unmarried person had been only half a person, who now becomes whole. A new king turns from a private individual to personifying the community as a whole. A convert is a completely new person.
Maharal (Chidushei Agados Vol. 1, p. 138, to Yevamos 63b): Cheit (sin) is an expression of Chisaron (lacking, shortcoming). When a person marries, he becomes whole, and his sins are forgiven. (EK)
Rashi writes: "That is why she was called 'Machalas' (28:9), for her sins were forgiven." But this would apply to any marriage; why is the name Machalas written specifically here?