What does the Pasuk mean when it says "u'Fanisi aleichem"?
Rashi: It means that Hashem will turn from all His other activities in order to reward us. 1
Targum Yonasan: It means that Hashem will turn from rewarding the nations in order to reward us.
Rashi: This can be compared to a king whose hired workers - except for one - did not perform their tasks properly. When they came to receive their remuneration, the king turned to the faithful worker and said to him 'My son; all your co-workers who did little work, I will pay a small wage; but with, I have to make a large reckoning!' And so it is here; Hashem will quickly pay of the nations of the world, but when it comes to Yisrael, He will turn aside from all His occupations, because He needs time to pay them for all their good deeds (Sifsei Chachamim).
What is the difference between "ve'Hifreisi eschem" and "ve'Hirbeisi eschem"?
Rashi: "ve'Hifreisi eschem" refers to increased numbers; "ve'Hirbeisi eschem", to stature. 1
Ramban: "ve'Hifreisi eschem" means 'You will all bear children' (that not a man or woman among you will be barren); "ve'Hirbeisi eschem," 'You will bear many children' (that no children will die at childbirth), and you will live long, so that you will become a vast nation.
See Sifsei Chachamim.
Which covenant is the Torah referring to?
Rashi: It is referring to a new covenant that Hashem will set-up with Yisrael, 1 not like the one at Har Sinai, which they abrogated.
Seforno: It refers to the covenant that Hashem made with Avraham - 'to be for him a G-d and his children after him', 2 which He will establish with them on their own merit, 3 not solely on the merit of the Avos. 4