1: The fact that Rav Yitzchak feels that the Torah should have started from החודש הזה לכם, the mitzva of the new moon that appears in Shmos , means that this is the beginning of the Torah in pnimius,
This is also understood in the context of the dispute of Rabbi Eliezer (Tishrei) and Rabbi Yehoshua (Nisan) re: the creation of the world. Chassidus explains that the machshava for the creation was in Nisan but the actual physical creation in Tishrei. So, too, re: the Torah; the external reality of Breishis bara is relevant to Tishrei and the pnimius and ultimate intention of Hachodesh hazeh lochem is relevant to Nisan.

2: Differences between Tishrei - milemata lema'ala and gvul and Nisan - milema'ala limata and beyond gvul.

3: Machlokes between Rabbi Eliezer and Rabbi Yehoshua re: future redemption.
Rabbi Eliezer: Tishrei and dependent on tshuva. Rabbi Yehishua: Nisan and not dependent on tshuva.
Question: We paskan that the future redemption is relative to Nisan (Rabbi Yehoshua) and yet that it is also dependent on our tshuva (Rabbi Eliezer).

4: This is understood by quoting the medrash of the three levels of our relationship to Hashem: biti, achosi, emi; daughter, sister, mother which correspond to Pesach, Shavuos and Sukkos. The advantage of the level of biti (relative to Nisan) is the complete bittul expressed therein. This allows for the revelation of etzem, unique to the future redemption also relevant to Nisan.

5: The unique nature of the avoda of this generation is bittul. This allows us to withstand even the greatest challenges (al yivosh mipnei hamaligim) as our avoda is beyond limitation in that it is a function not of our own sechel and middos but rather of Hashem's greatness. Biti is lower than the other two levels only in times of Beis haMikdash when elokus is revealed but in golus the bittul of biti is higher.

6: This helps explain our question in 3. The tshuva of ikvasa deMoshiach is a Nisandik tshuva, unlimited in its scope.. It is avoda from below but predicated on fact that Torah promises that we will do tshuva before Moshiach.

7: Therefore tshuva before Moshiach has the advantages of Tishrei (avoda from below) and Nisan (draws down unlimited light). The intention of the creation of the physical in Tishrei is the revelation of the unlimited light of Nisan in that lower physical realm.

8: Therefore the true beginning of the Torah is Hachodesh hazeh lochem but this level of light must be revealed in 'Breishis bara'. Thus, Breishis is discussed first and then the pnimius of Hachodesh hazeh lochem is revealed and permeates it.

9: The Rebbe discusses the two levels of creation, כ"ה אלול and the creation of man on א' תשרי. Our tshuva affects and perfects the reality of the first of Tishrei and the commensurate revelation from above of Moshiach elevates and permeates the reality of כ"ה אלול.