1. לא בצדקתך ובישר לבבך אתה בא לרשת את ארצם כי ברשעת הגוים האלה ה אלקיך מורישם מפניך ולמען הקים את הדבר אשר נשבע יהוה לאבתיך לאברהם ליצחק וליעקב. If the nations sinned and Klal Yisrael sinned, why are they more deserving to obtain the land? And what is the combination of the רשעת הגוים and the שבועה לאבות (see Or HaChayim.) The two questions answer each other. It is the bris to the Avot that defines the sins of Klal Yisrael as different from the other nations. The sins of the other nations define their essence and therefore they are rejected. However, the sins of Klal Yisrael do not define their essence, instead, their essence is defined by the shavua to the Avot.
2. The possuk in Bo (12:23) says the commandment to place the blood on the Pesach on the doorposts and the very next possuk says ושמרתם את הדבר הזה לחק לך ולבניך עד עולם. The Or Hachayim asks the simple read of the possuk is that the command to place the blood is everlasting so why do we not do that? In a sicha (Shavuot 5634) the Rebbe says that we do fulfill the commandment of the blood albeit in a different form. The mitzvah of the blood was to act as a protection against the משחית and we have the mitzvah of mezuzah which has the same effect. How does the mezuzah have this effect?
The word מזוזה contains the words זו and זה. These words are used to describe the close connection Klal Yisrael and Hashem have, עם זו יצרתה לי and זה קלי ואנוהו. The mezuzah indicates that we designate our living place our lives to Hashem and Hashem in turn watches over us.
The Parsha Va'eschanan speaks to the saga of Klal Yisrael as they are about to enter Eretz Yisrael and Ekev hints to the saga of the Jews toward the עקב of time, during the golus era. Vaechanan can be described as the parsha of seeing, when things are clear that Hashem is leading the day. The parsha of Ekev speaks to the times when G-d's presence is not as clear and when must find Him, more in line with the sense of listening. This may explain why Ekev places greater emphasis on reward and punishment, rain in its proper time, and other practical consequences of mitzvah observance. In the time when Hashem's Presence is not always openly visible, people need to recognize Hashem through the events of everyday life. (see L.S. Ekev volume 9.)
The parsha of mezuzah appears in both parshios. It demonstrates our commitment to Hashem in times of זה as in זה קלי ואנוהו when Hashem's presence can be seen directly and in times of זה+זו=25, of כה, when the vison of Hashem is obfuscated.
(See Sicha Shavuot 5654, 15 Shevat 5654, maamer Ekev 5629)