the mitzvah of kevurah and sarah's burial

Avraham approached the people of Bnei Cheis and asked for a burial plot for Sarah, ‘v’ekbira es meisi milfanei’, so that her body could be removed from before him for burial.
1) Ramban infers from the words “before him” that had Avraham not been able to purchase a burial plot, he would have carried Sarah’s body with him interred in a coffin. The Chavel edition of the Ramban notes that this contradicts the Ramban’s position in Toras haAdam that burial must be done in the ground. The Klei Chemdah suggests that there are 2 reasons for burial in the ground: a) because the human body comes from the ground and needs to be returned to its rightful place; b) so that the body does not suffer bizayon, disgrace, by lying in the open. The Ramban took Avraham’s words to mean that even if he was denied a burial plot in the ground, he would still fulfill the aspect of the mitzvah of kevurah of preventing the body from coming to disgrace by interring the body in a coffin.
2) The gemara (Sanhedrin 46b) debates whether the reason for the mitzvah of burial is “m’shum bizyonei”, so that the body is not disgraced, or “m’shum kapparah”, so that the dead receives kapparah by the body decomposing. The nafka minah would be if a person requests that his body not be buried – if the reason for burial is to avoid disgrace, to some degree it is the family who suffer disgrace by their relative not being buried, and a person has no right to inflict this situation on others; however, if the purpose of burial is for person kaparah, a person has a right to forgo the process. The gemara asks if the reason is kaprah, why do tzadikim who have not sinned need to be buried? The gemara answers “ain tzadik b’aretz”, everyone needs some degree of kaparah. The Yismach Moshe writes that given Rashi citation of the Midrash that Sarah was untainted by sin like a 20 year old, she indeed was a perfect tzadeikes and perhaps did not need the kaparah of kevurah. Therefore, Avraham pleaded for a burial plot “milfanei”, so that his bizayon at not burying his wife should be removed from before him, but not for her sake, as she transcended the need for kaparah.