Rashi #1: [Avraham] feared lest he be punished for those whom he killed. Hashem told him "Al Tira Avram" (Bereishis 15:1) - He will not retract from the good that He said about (to do to) you.

Rashi #2: Kingship will be from you (David) forever.

Radak: Hashem's word is His Shevu'ah.

Malbim: Hashem swore to give to you kingship; He will not retract due to sin.

Rashi #1: Kehunah and kingship will come from you (Avraham). Your children will inherit the Kehunah and kingship that were given to Shem, your ancestor. It is as if it says Al Divras Malki Tzedek, his appointment; the Yud at the end of Divrasi is extra, like in "Rabasi Am" (Eichah 1:1). 'Kohen' implies Kehunah and authority, like "u'Vnei David Kohanim Hayu" (Shmuel II, 8:18).

Rashi #2: This refers to Kehunah above that of Malki Tzedek, i.e. kingship, which is acquired with 30 attributes, more than Kehunah Gedolah (which is acquired with 24). Even though Malki Tzedek was also a king, it is not an esteemed kingship compared to Yisrael.

Radak (1, citing Nedarim 32b): Kehunah was proper to come from Malki Tzedek (Shem) - "v'Hu Chohen l'Kel Elyon" (Bereishis 14:18). He was a Kohen, but his seed was not. Hashem transferred it to Avraham Al Divras Malki Tzedek" (due to Shem's words; the Yud is extra). Shem blessed Avraham before Hashem ("blessed is Avraham... and blessed is Hashem" - ibid., 19-20) .

Radak: It says "l'Olam", for Sha'ul's kingship was not forever. I chose you to be king forever, for the Davar that you are Melech ¹ Tzedek - "va'Yhi David Oseh Mishpat u'Tzedakah l'Chol Amo" (Shmuel II, 8:15).

Malbim: David is called Malki Tzedek, like the first king of Yerushalayim - "u'Malki Tzedek Melech Shalem" (Bereishis 14:18). [Malki Tzedek] used to be the name of kings of Yerushalayim. You will be Kohen on My Devir, i.e. the Mikdash and Devir (Kodesh ha'Kodoshim). "Tzedek" also teaches that he is a Tzadik in His eyes; He pardoned his sin.