What are the implications of "Lema'an Lo Yelamdu eschem ... "?
Rashi #1 and Ramban #1 (citing the Sifri): This implies that in the event that they do Teshuvah and convert (and accept the seven Mitzvos B'nei No'ach - Ramban), one is permitted to accept even the seven nations. 1
Ramban #2 (citing Rashi #2 in Sotah, 35b): Teshuvah is only acceptable with regard to the other nations or Cana'anim in Chutz la'Aretz, but not with regard to Cana'anim in Eretz Yisrael 2 (whose Teshuvah is based on fear and is not genuine ? (Tosefta Sotah, 8). 3
See Sifsei Chachamim. Refer also to 20:16:1:2* & 20:18:151:1 and note.
See Torah Temimah, note 54, citing the Tosefta ibid.
This answers the Kashya of the Ramban on Rashi who queries Rashi's distinction between the Cana'anim in Eretz Yisrael and the Cana'anim in Chutz la'Aretz.
Why does the Torah add the phrase "Ve'chatasem la'Hashem Elokeichem"?
How will we reconcile the fact that if they do Teshuvah, we may accept even the seven nations with the Pasuk "Lo Sechayeh Kol Neshamah" (in Pasuk 16)?
Refer to 20:16:1:1*.
Moshav Zekenim: Pasuk 16 is speaking when they do not want to repent; if they do 1 we may accept them. 2
Oznayim la'Torah (citing the Tosefta Sotah, 5: One is only permitted to accept Cana'anim who live in Chutz la'Aretz if they do Teshuvah, 3 but not in Eretz Yisrael, where "Lo Sechayeh Kol Neshamah" is unconditional - since their Tshuvah is out of fear and is not genuine.
Moshav Zekenim: And the reason that Yehoshua was angry with the Giv'onim was because they did not want to become complete Yisre'elim, only not to serve idolatry.
Refer also to 20:18:1:1 & 20:18:1:2*.
See also Torah Temimah at the beginnning of Ki Seitzei, citing Sotah, 35b, and note 69.
