What is this ascension referring to?
Rashi #1: To receive the second set of Luchos; and the Torah mentions it here to teach us how long he remained there. 1
Rashi: Which it did not do above - at the beginning of the Perek.
What are the implications of the words "ka'Yamim ha'Rishonim"?
Rashi #2: To teach us that these forty days were days of goodwill, just as the first ones were. 1
Ramban (citing the Seider Olam): To teach us that, during these forty days, Moshe Davened to Hashem with the thirteen Midos ('Hashem Hashem') just as he did during the first forty days. 2
What are the implications of "Lo Avah Hashem Hashchisecha"?
Ramban #1: Hashem did not want you not to come and inherit Eretz Yisrael (immediately), so as not to sin like the Meraglim sinned and cause Yisrael to be destroyed. 1
Ramban #2 (According to Kabalah): It implies that Hashem (the G-d of Mercy) does not want destruction to emanate from Him, and that His great Name should not accompany you. 2