1.

The Tiferess Tziyon (cited by the Metivta Gemara) answers that had we had only the verse in Bamidbar 24:21, we might have thought that this verse refers to two subjects: "Eitan Moshavecha" is one matter, and "ba'Sela Kinecha" is a separate matter. There would be no proof that there is any connection between the two.

In contrast, the same mistake could not have been made with the verse in Michah 6:2, apparently because the word "Eitanim" used there is merely an alternative way of saying "Harim."

2a. I would like to take this idea a little further and explain the matter a bit more.

In fact, it is not at all clear that the only meaning of the word "Eitan" used in Bamidbar 24:21 is "hard." The Seforno there explains the verse in a different way, according to the Peshat. He writes, "You settled with Israel in an Eitan land, in the wilderness, an unsown land." It appears that the Seforno understands the word "Eitan" to mean "old." The idea is that the "Keini" and their leader, Yisro, were together with Yisrael in the ancient desert.

My proof that "Eitan" can also mean "old" is from Melachim I 8:2, "b'Yerach ha'Eitanim," or "the month of the Eitanim," which is Tishrei. Targum Yonasan writes that this is the "ancient month." (This is because before the Bnei Yisrael went out of Egypy, Tishrei was considered the first month.) See also Sotah 36b: "she'Shavah Kishto l'Eitano" -- that the Keshet of Yosef returned to its original state.

2b. Therefore, "Eitan Moshavecha" could have been translated as, "You have been settled from ancient days," and the other words in the verse, "ba'Sela Kinecha" could not prove otherwise, as the Tiferes Tziyon writes.

This is why the Gemara cites the other verse in Michah. It is a more conclusive proof that Eitan means hard.

Kol Tuv,

Dovid Bloom

a. Of course, the idea that Eitan means "old" is the second opinion on Sotah (beginning of 46b). We see this from Yirmeyahu 5:15, where "Goi Eitan" means an "ancient nation." Therefore, the first opinion on 46b needs to bring an additional source from Michah that "Eitan" means "hard," because otherwise one could have said that "Eitan Moshavecha" means that the settlement of the Keini is old and well-established, as I wrote in my first reply.

b. However the first opinion can refute the proof of the second opinion from Yirmeyahu 5:15 and state that "Goi Eitan" also means a strong, powerful nation. In fact the Mechilta (printed in the Malbim Chumash) on Shemos 14:27 proves from the verse in Yirmeyahu that "Eitan" means "powerful," and indeed Rashi in Yirmeyahu also defines it as "Takif" (powerful).

Kol Tuv,

Dovid Bloom

1. In the first Lashon in the Gemara, the Tana cites the verse from Bamidbar because it is better, if possible, to cite a verse from Chumash rather than a verse from Navi. However, the verse from Chumash is not sufficient alone because one could say that it means "old." In contrast, the verse in Michah could not be rendered as "old" and can mean only "hard." Therefore, the clear meaning of the word in Michah reveals that the ambiguous word in Bamidbar also means "hard," so in the end we have a Chumash word "Eitan" which means "hard," which is the strongest possible conclusion.

2. The second Lashon in the Gemara maintains that Yirmeyahu 5:15 is not ambiguous at all. This is because this verse indicates that an "Eitan" nation is the same as a nation "me'Olam," i.e. an ancient nation.

Ketivah v'Chatimah Tovah,

Dovid Bloom