What did Bil'am mean when he said "Ki Lo Nachash be'Ya'akov ... "?
Rashi: 'They deserve to be blessed because they do not harbor diviners and sorcerers'.
Ramban: Hashem's B'rachos are bound to materialize, seeing as they do not harbor diviners and sorcerers.
Seforno: The reason that 'Hashem took Yisrael out of Egypt and gave them the strength of a Re'em is because they seek Hashem and do not indulge in witchcraft.
Rashbam: 'Because they (Yisrael) are not diviners and sorcerers like we (Bil'am and Balak) are, nor do they consult Ov and Yid'oni (like we do)'.
Da'as Zekenim, Hadar Zekenim: Nachash is an expression of testing; the Targum of "Nichashti" (Bereishis 30:27) is Naseisi, Do not test if He will curse them; surely, He will not curse them. If you will say that He will not curse them, but one can harm them through witchcraft
What did he mean when he said "ka'Eis Ye'amer le'Ya'akov ... "?
Rashi #1: 'There will come a time such as this 1 when their closeness to Hashem will be revealed, when they sit before Him - closer than angels 2 studying Torah and the angels will ask them what G-d has wrought'.
Rashi #2, Ramban #1 (citing Rashi) and Rashbam: Whenever they need to know what Hakadosh-Baruch-Hu has decreed (or to know what will happen some time in the future - Rashbam) they have access to the information by means of the prophets (or the Urim ve'Tumim - Rashi). 3
Ramban #2: Since Balak sent for him to perform sorcery against Yisrael, Bil'am is now stating that neither divination nor sorcery are effective on Yisrael, whether for good or for bad, because both good and bad come on Yisrael directly from Hashem. 4
Targum Yonasan: At this (Da'as Zekenim, Hadar Zekenim - at a future) time it will be told to Yisrael how praiseworthy are the miracles and the wonders that Hashem performed for them. 5
See Sifsei Chachamim.
Rashi in Sukah, 45b: Because Tzadikim are closer to Hashem than angels.
Rashi and Seforno: And they have no need of divination or sorcery.
Ramban: Due to the fact that Yisrael are not under the jurisdiction of the celestial powers (as are all the other nations) but under the jurisdiction of Hakadosh-Baruch-Hu Himself - as Moshe Rabeinu says in Devarim, 4:19, 20. Refer also to Vayikra, 18:21:1:1.
See Na'ar Yonasan.
Why did he mention the two names separately, and then together
Kol Eliyahu, Divrei Eliyahu: Just like one who prays for someone must mention his name, also one who wants to harm through Nichush and Kishuf. He could not mention all 600,000 individual names, so he chose Yakov, the choice of the Avos, from whom came the Shevatim