There is a Talmudic maxim (Yevamos 97b) that "a non-Jew who converts to Judaism is legally considered newly reborn," and is no longer the person he was with the relatives he used to have. In his commentary on Avodah Zara (63b), the Chasam Sofer writes that he was troubled his entire life at his inability to locate a source for this ruling which the Talmud in many places seems to take for granted.
The Meshech Chochmah suggests that this rule may be derived from our verse (Deut. 5:27). Moshe's father Amram was one of the greatest men of his generation and was married to his aunt Yocheved, a marriage which is forbidden to Jews but permitted to non-Jews. If one of the leaders was married to his close relative, it is reasonable to assume that many other Jews did likewise and married family members who weren't forbidden to non-Jews.
After the giving of the Torah, God instructed Moshe to tell the people to return to their tents. The Talmud (Moed Katan 7b) understands this as a reference to their wives. Although they were required to abstain from marital relations for three days prior to the giving of the Torah in order to receive it in a state of spiritual purity, they were now permitted to resume normal family life.
However, at this point, those Jews who were married to relatives to whom they were now forbidden by the Torah should have been required to divorce them. If God told Moshe to permit all of the Jews to return to their wives, their conversion must have made them as if they were reborn and no longer related to their wives. This permitted them to remain married, and from here we may derive the source for the law which the Chasam Sofer sought.
* * *
200 DAYS OF PRAYER
Moshe petitioned God 515 times to rescind His decree and allow him to enter the land of Israel (Deut. 3:23). The Midrash (Yalkut Shimoni Devarim 940) teaches that this is hinted to in the word V'etchanan - "and I beseeched" - which has a numerical value of 515.
The P'nei Yehoshua (Brachos 32a) writes that this figure may also be mathematically derived. Rashi writes that Moshe began to entreat God after conquering the lands of Sichon and Og. This area, where the tribes of Reuven, Menashe and Gad dwelled, would later possess some of the holiness of the land of Israel. Because Moshe was permitted to enter this region, he thought that perhaps God had revoked His oath prohibiting him from entering the land of Israel.
Even before the actual military battle, Moshe know that the Jewish people would emerge victorious. God told him not to fear Sichon and Og (Deut. 2:31, 3:2) since He had already delivered the Heavenly angel in charge of their lands into Moshe's hands. The Talmud relates (Bava Basra 121a, also see Rashi 2:17) that God told him this on the 15th of Av. On that day, the last of those sentenced to die in the wilderness due to the sin of the spies passed away, and the Jews once again merited God's prophecy.
From 15 Av until the day of Moshe's death, 7 Adar, there are 200 days. If Moshe implored God during each of the three daily prayers, he would have petitioned a total of 600 times. However, it is forbidden to pray for one's personal needs on Shabbos (Mishnah Berurah 288:22). Subtracting the prayers which he wasn't permitted to say on Shabbos, of which there were 28 during this period, leaves a total of 516 prayers.
However, prophecy didn't return to Moshe on 15 Av until the morning, leaving him without a reason to beseech God during that day's evening prayers. From the morning of 15 Av until his death on 7 Adar at the time of Mincha (Tosefos - Menachos 30a d.h. mi'kan), Moshe prayed for the nullification of the decree precisely 515 times.
* * *
THE MITZVAH AGENT
Many mitzvot may be performed by appointing an agent to do so on his behalf, such as betrothing a woman (Talmud - Kiddushin 41a). Can the mitzvah of honoring one's parents (Deut. 5:16) be done via an agent?
Rabbi Tzvi Pesach Frank (Har Tzvi - Parshas Yisro) quotes Rabbi Yaakov Klemes as ruling that the mitzvah of honoring one's parents cannot be performed via an agent. As proof, he cites the Talmud (Kiddushin 32a) which discusses what a child should do if his father asks him to bring him a glass of water at the same time that he also has another mitzvah, such as burying the dead, to perform. One opinion maintains that the son should fulfill the other mitzvah at the expense of his father's drink, as both the father and the son are obligated to honor God by performing mitzvot.
A second opinion argues that this is not always the proper course of conduct, explaining that if the other mitzvah can be performed by somebody else, the son should allow the other person to do the mitzvah and he should tend to his father's request. From the fact that the Talmud only discusses the scenario in which the other mitzvah can be performed by somebody else, but doesn't entertain the possibility that the father's drink can be brought by an agent, Rabbi Klemes concludes that the mitzvah of honoring one's parents cannot be performed by an intermediary. However, Rabbi Frank adds that this argument is not conclusive and the subject needs further study.