Why would those who bless Avraham be blessed themselves, and those who would curse be cursed?
Maharal (Netzach Yisrael Ch. 59, p. 218): Avraham was the initiator of a new stage in history, the 2,000 years of Torah; refer to 11:32:2.2:2. Anyone who would deny this new reality would himself fade away from reality; whereas one who would embrace it would bring blessing upon himself.
Why did HaSh-m switch the order from "I will bless those who bless you," to, "Those who curse you, I will curse"?
Kli Yakar: HaSh-m rewards already for a person's intention to perform a Mitzvah, but punishes only when he actually transgresses. Consequently, He is informing Avraham that He will bless those who intend to bless him even before they have done so, but He will only curse those who have actually cursed him.
Kol Eliyahu, Divrei Eliyahu: An Ashir's blessing is greater than an Ani's blessing, and his curse is less than an Ani's. Those who bless you, I will bless them beforehand, so they will be Ashirim when they bless. Those who curse you, I will curse them afterwards, so they will be Ashirim when they curse.
Why does the Torah repeat that all the families of the earth will be blessed through Avraham? (Verse 12:2 already stated, "be a blessing.")
Rashi: It means that people will say to their children, "Be like Avraham!"
Ramban: Not only the people of Kena'an, but also the people all over the world will be blessed due to Avraham.
Targum Onkelus and Targum Yonasan: All the people in the land will be blessed due to Avraham Avinu.
Rashbam, Da'as Zekenim: The Torah means that all the families of the world will (want to) join his family (from the word "Mavrich" - to graft).
To whom does "Avarechah Mevarechecha" apply?
Da'as Zekenim (based on Chulin 49a): The Kohanim will bless Yisrael, and HaSh-m will bless the Kohanim.
Yitzchak first cursed those who will curse [Yaakov], and then blessed those who bless him, for Tzadikim initially have afflictions, and have serenity at the end (Rashi to Bereishis 27:29). Why did HaSh-m do oppositely?
Moshav Zekenim #1: HaSh-m's Berachah is unlike a person's Berachah.
Moshav Zekenim #2: Here in any case the end is serenity - "v'Nivrechu Vecha."
Moshav Zekenim #3: HaSh-m wanted to put "Avarechah Mevarechecha" next to "ve'Heyei Berachah (12:2)."
What is the benefit of cursing one who curses Avraham? It would be better to nullify the curse!
Moshav Zekenim: Some people have a Mazal that their curses take effect even on a Tzadik. Now, all will fear to curse Avraham.
Refer to 12:3:1:2.