Why does the Torah switch from Elokim to the Name Havayah?
Ramban: Hashem is about to inform No'ach (in Pasuk 3) that all the animal world would live with him in the ark. By mentioning the Name Havayah however (the Name that denotes mercy and that is always used in connection with Korbanos 1 ), He is hinting to him that on the merit of the Korban that he would bring upon exiting the ark, a. the entire world would continue to exist. And b. He would never again destroy the world.
What was the point of telling No'ach that he was a Tzadik?
Ramban and Seforno: Hashem was telling No'ach that he alone (and not his family) 1 was a Tzadik worthy of saving the entire world.
Why does it then omit the adjective 'Tamim' that it inserted at the beginning of the Sedra?
Rashi: Because one never mentions all a person's praises in his presence.
Having informed No'ach that his family would enter the ark, why repeat it here?
Ramban: In the Pesukim prior to Sheini, which were said to No'ach years before the Flood, Hashem was telling him what would happen at the time of the Flood. Here, He is giving him instructions a week before the Flood (on the tenth of Mar-Cheshvan).
רש"י: שאומרים מקצת שבחו של אדם בפניו וכו': למה כך היא ההנהגה?
מהר"ל (נתיבות עולם, נתיב התוכחה פ"א עמ' קצ"א ד"ה ואמר): באמירת כל שבחו נראה כדברי חנופה, אבל את מקצת שבחו יש לספר להראות שהוא בעל מעלה בעיניו, כדי להביא אהבה ביניהם.