Who or what in the world is Yeshurun?

Sometimes I'll start a post, and for several days/weeks/months will not be able to finish it. By the time I do get around to it, new info will have come to my attention and a re-write will be needed (sometimes that's the cause of the laziness of not finishing the post). That's the case with this post.

Original beginning: Spoiler warning: There will not be a satisfying chiddush at the end of this post. Just some head-scratchers, at best.

New beginning: Please partially disregard the previous beginning.

Yeshurun! So majestic, so inspiring, so...actually, this one isn't so bizarrely-named. Yeshurun is one of the names of the People of Yisrael. Surprisingly, this is one of the few points that just about every commentator, both Jewish and otherwise, agree on. Take that, whoever invented "two Jews, three opinions"! *ahem* Anyway, "Yeshurun" appears a total of four times in Tanach:

1. "So Jeshurun grew fat and kicked—You grew fat and gross and coarse. He forsook the God who made him and spurned the Rock of his support." - "וַיִּשְׁמַן יְשֻׁרוּן וַיִּבְעָט שָׁמַנְתָּ עָבִיתָ כָּשִׂיתָ וַיִּטֹּשׁ אֱלוֹק עָשָׂהוּ וַיְנַבֵּל צוּר יְשֻׁעָתוֹ." (Devarim 32:15).
2. "Then He became King in Jeshurun, when the heads of the people assembled, the tribes of Israel together." - "וַיְהִי בִישֻׁרוּן מֶלֶךְ בְּהִתְאַסֵּף רָאשֵׁי עָם יַחַד שִׁבְטֵי יִשְׂרָאֵל" (Devarim 33:5).
3. "O Jeshurun, there is none like God, riding through the heavens to help you, through the skies in His majesty." - "אֵין כָּאֵל יְשֻׁרוּן רֹכֵב שָׁמַיִם בְּעֶזְרֶךָ וּבְגַאֲוָתוֹ שְׁחָקִים." (Devarim 33:26).
4. "Thus said the LORD, your Maker, your Creator who has helped you since birth: fear not, My servant Jacob, Jeshurun whom I have chosen." - "כֹּה אָמַר ה' עֹשֶׂךָ וְיֹצֶרְךָ מִבֶּטֶן יַעְזְרֶךָּ אַל תִּירָא עַבְדִּי יַעֲקֹב וִישֻׁרוּן בָּחַרְתִּי בוֹ." (Yeshayahu 44:2)

I mean, all in all, it makes sense why everyone thinks Yeshurun refers to Yisrael. And I'm not disputing that, Hashem forbid.

What I'm wondering is where the name comes from. Unlike the names "Yaakov" and "Yisrael" which we know where they come from (Yaakov - the name his parents gave him; Yisrael - the name the angel gave him), or even, say, "Jews" (which comes from the Land of Judah/Judea), there is no record of where the name came from and why it is used to refer to Bnei Yisrael.

And so, I did some digging. Ben Tzion Luria, having been himself, came up with a creative idea. In his essay "פרקי קדומות" (Ancient Chapters) in Sefer Yosef Breslevi (Jubilee Volume for Yosef Breslevi), he suggests that at least some of Shirat Ha'azinu (the song chapters in Devarim where the term first appears) is actually a remnant of an ancient Hebraic war-epic-poem. When he says "Hebraic", he's referring to the ancient Hebrews. He thought that the ancient Hebrews had once been a nation, and that nation was called...(drumroll, please) "Yeshurun"! And so, since Yisrael are the biological and spiritual descendants of Yeshurun, the title was passed over to them. The various verses in the Torah that mention Yeshurun refer to events in the Yeshurunic people's past. These now come to teach Yisrael certain things.

Very cool idea. One problem: Zero evidence. Luria insisted that the ancient Hebrews used to be a nation unto themselves. There is no mention of this in Tanach. There is only one place (if I recall correctly) in Tanach where the term "Hebrews" refers to a group that consists of not just the particular lineage that fathered Yisrael, and can also be understood to referring to some organized ethnic collective-clan-tribe-thing, and that's in Beresheet 40:15: "For in truth, I was kidnapped from the land of the Hebrews; nor have I done anything here that they should have put me in the dungeon.”" As I mentioned in a recent post, there are different theories about what this "land of the Hebrews" was, and I myself am working on a theory as well, but that's not important right now. What's important is that in the only place in which such a collective appears, they are simply called "Hebrews". Nothing more and nothing less.

It was about this point that I was planning to end the previous version of this post. However, Sukkot and stuff like that got in the way, so to speak, which was lucky, because a bit of a breakthrough came on Simchat Torah. On Simchat Torah I was in yeshiva, and one of the rabbis who gives classes once a week by us also came to the yeshiva. His name is Rabbi Uzi Bienenfeld, and he's very knowledgeable in Tanach. I asked him about this. He suggested two directions. Before that, I'll say that both directions hold that Moshe was the one who thought up of the name. Now, for the directions:

1. Yeshurun - ישרון - has a ון (a Vav and a Nun) at the end. This signifies a condensation, as well as turning the word into future tense. To explain what this means, he used the verse "Then He became King in Jeshurun, when the heads of the people assembled, the tribes of Israel together." - "וַיְהִי בִישֻׁרוּן מֶלֶךְ בְּהִתְאַסֵּף רָאשֵׁי עָם יַחַד שִׁבְטֵי יִשְׂרָאֵל" - what this means is that in the future, when the People of Yisrael unite (that's the condensing part), then they will merit to have a king. Indeed, that's what happened (and see this post as well). It was only through tribal unity that a king could be chosen for the nation.
2. The Vilna Gaon wrote that Yeshurun - ישרון - comes from the root שיר - "song", because the first truly significant and powerful moment that Moshe experienced where Yisrael were fully a nation was during Shirat Hayam (the Song of the Sea) - it was through song that they expressed their feelings after the great miracle that had happened to them. It was really only then that they were truly free of the Egyptians and could assume a national identity. For this reason, Moshe referred to them as those that sing. This ties into the previous point that says that it's in future tense: Moshe is telling them to keep on singing, because that's a core part of their identity. What's particularly special is that he's telling them this during a song (Shirat Ha'azinu).

(Yeshurun the bull from "Parashat Baba" by Shay Charka)