To what days is Hashem adding?
Radak citing Yevamos 49b #1: He added back what was initially allotted for him, and it would have been deducted due to his sin (of not marrying) 1 .
Radak citing Yevamos 49b #2: He added to what was initially allotted for him.
Radak: I explained this in Sefer Shmuel. Refer to Shmuel I, 26:10:5:1 and the note there.
Why does it say both "I will save you from Melech Ashur" and "I will protect the city"?
Malbim: The latter teaches that the salvation will be miraculous.
Why does it say "Lema'ani u'Lema'an David Avdi"?
Malbim: Based on the generation, a natural salvation would have sufficed. For Kidush Hashem (Lema'ani) and to make sprout the horn of David, it will be miraculous.
Brachos 10b: If one requests something in his own merit, it is attributed to another's merit 1 . Chizkiyah said "Zechor Na Asher His'halachti Lefanecha" (verse 3), and Hashem saved him "Lema'ani u'Lema'an David Avdi."
10b: If one requests in another's merit, it is attributed to his own merit. Moshe said "Zechor l'Avraham l'Yitzchak ul'Yisrael Avadecha", and it says "va'Yomer Lehashmidam Lulei Moshe Bechiro Amad ba'Peretz" (Tehilim 106:23).
