What is "Memis u'Mechayeh"?
Radak #1: It is to kill, and extend life - the opposite of Memis.
Radak #2: It hints to reward of Tzadikim and punishment of Resha'im.
Radak #3: She spoke via prophecy, or tradition, about Techiyas ha'Mesim in the future. Malbim - since she mentioned the other two 'keys' that Hashem does not give to a Shali'ach (birth and rain (income)), she mentioned also the third (Techiyas ha'Mesim).
Bi'ur ha'Gra: This is like "Nafalti Kamti" (Michah 7:8). Hashem took Moshe's life, and aggrandized Yehoshua.
Aderes Eliyahu #1: Moshe and his generation, who were "Chayim Kulchem" (Devarim 4:4), died. Hashem put an Atzeilus (semblance of a copy) of its light on the leaders of all generations ("Dor Holech v'Dor Ba" - Koheles 1:4).
Aderes Eliyahu #2: Memis - in the mourning over Moshe, 3,000 Halachos and 1,700 Kal v'Chomerim were forgotten. U'Mechayeh - Osni'el ben Kenaz recovered them.
Malbim: He kills the sons of Rabas Banim, and sustains the sons of the barren. 1
Malbim: This and the next verse teach that the changes mentioned in the last three verses (between birth and death, wealth and poverty, and strength and weakness) all come from Hashem.
What is "Morid She'ol va'Ya'al"?
Radak #1: He brings [close to death] and saves.
Radak #2: Hashem afflicts, and gives relief.
Bi'ur ha'Gra: He delivered then into the hands of Yavin Melech Chatzor, and established Shoftim to save them.
Aderes Eliyahu: She'ol refers to exile; Bavel is called She'ol (Yevamos 17a). (Va'Ya'al is redemption from exile.)
