Who were the Yevusi?
Rashi: It is not the name of the nation. Rather, they were Pelishtim of a region (Migdal David) called Yevus.
Radak: It is not the name of the nation. Rather, they were descendants of a Pelishti, a descendant of Avimelech, named Yevus.
Metzudas David: It is the Yevusi nation.
Why could they not dispossess the Yevusi there?
Rashi, citing Sifri Re'eh 12:17: Yehudah had the power to do so, but they were not allowed, due to Avraham's Shevu'ah 1 to Avimelech. 2 When Yehudah conquered Yerushalayim, they did not dispossess the people there.
Radak: The fortress was strong, and Hashem caused that Yehudah could not conquer it until David, so it would be called on his name. "Ad ha'Yom ha'Zeh" is when Yehoshua wrote this.
Radak citing Pirkei d'R. Eliezer 36: The Shevu'ah was written on copper images. David removed them, and afterwards conquered the tower and bought the Yevusi city for Yisrael from Aravna (Divrei ha'Yamim 1 21:25).
Rashi (Shoftim 1:21): The Shevu'ah was "Li ul'Nini ul'Nechdi" (Bereishis 21:23); Avimelech's grandson was still alive. (How would they know this? They never asked from the Urim v'Tumim in Yehoshua's lifetime! Perhaps Hashem told Yehoshua. Or, Serach bas Asher had seen him before descending to Egypt, and recognized him (she was still alive in the 40th year in the Midbar
Why is there a Vav after the Yud in "Yachlu"?
Radak: It is read "Yuchlu", to teach that also later, they could not dispossess them (due to the Shevu'ah; refer to 15:63:2:1).
Why is this repeated in Shoftim (1:21), but there it says that Bnei Binyamin did not dispossess them?
Radak (Shoftim 1:21): Yerushalayim is between Yehudah and Binyamin.
Malbim (Shoftim 1:8): Melech Yerushalayim was the first of the five kings to fight against Giv'on. Yisrael killed him. They surely thought to capture his city, like the cities of the other four kings that they killed. Yehudah fought, for a strip including [part of] Yerushalayim is in its border. They could not dispossess the Yevusi there; they remained "Ad ha'Yom ha'Zeh", i.e. until the end of Yehoshua's lifetime; he wrote his Sefer. After Yehoshua died, there was war between them, and they captured it and burned it, to punish them for their rebellion. However, they did not capture the fortress called later Metzudas Tziyon, until the days of David. The Yevusi were also there. Binyamin did not fight them at all. In Shoftim, it says that they remained with Binyamin "Ad ha'Yom ha'Zeh", i.e. until the end of Shmuel's lifetime; he wrote Sefer Shoftim.
What is the meaning of "Ad ha'Yom ha'Zeh"?
Radak, Malbim: It is when Yehoshua wrote this.