Multiple Tribes of Binyamin?

Question

In the book "Sefer Yosef Breslevi", there's an essay by Yisrael Ben-Shem in which he raises multiple questions related to various appearances of people from the Tribe of Binyamin throughout Tanach. One of the questions he asks is about the phrase "שבטי בנימין" (the tribes of Binyamin) which appears twice in Tanach, once in Shoftim 20:12:

"וַיִּשְׁלְחוּ שִׁבְטֵי יִשְׂרָאֵל אֲנָשִׁים בְּכָל שִׁבְטֵי בִנְיָמִן לֵאמֹר מָה הָרָעָה הַזֹּאת אֲשֶׁר נִהְיְתָה בָּכֶם."

And once in Shmuel 1:9:21:

"וַיַּעַן שָׁאוּל וַיֹּאמֶר הֲלוֹא בֶן יְמִינִי אָנֹכִי מִקַּטַנֵּי שִׁבְטֵי יִשְׂרָאֵל וּמִשְׁפַּחְתִּי הַצְּעִרָה מִכָּל מִשְׁפְּחוֹת שִׁבְטֵי בִנְיָמִן וְלָמָּה דִּבַּרְתָּ אֵלַי כַּדָּבָר הַזֶּה."

He writes that only Binyamin is ever referred to as being made up of multiple tribes. The question is, of course, why? What does this mean?

He answers that indeed Binyamin was made up of two distinct ethnic groups which eventually merged: Tribe A: The descendants of Binyamin, natural Israelites; and Tribe B: A group called "Bnei Yamin" or "Bnei Yemini" which, besides being distinct phrases in Tanach, are also seemingly attested to having existed in a tablet discovered in the ancient city-state of Mari. According to him, this answer solves all of the other questions he raised, such as 'why did the people of Yavesh Gilad burn the bodies of Shaul and Yonatan if that wasn't Israelite custom?'

The classic commentators, from what I've seen, explain that the plural term refers to the families or clans (בתי אב/משפחות) of Binyamin - each family or clan within the tribe was also called a "tribe". Ben-Shem points out that this explanation would work, were this a one-time thing or if it was used to describe more than one tribe, but as it's used twice and both times referring to the same tribe - seems odd, to say the least.

Is there a more Orthodox/traditional answer to this problem?

Answer 1

Rashi on the verse in Shoftim says that the ten families of Binyamin were counted as separate tribes, so that Rachel could be said to be the ancestress of twelve tribes in her own right (those ten, plus Menashe and Ephraim).

(It is true that the sons of Yosef are never called שבטי יוסף collectively. However, they are referred to individually as שבט אפרים and שבט מנשה in numerous places, and of course for many purposes they do count as separate tribes.)

Also note Sotah 36b, that Yosef by rights should have had twelve tribes descend from him, but that he lost that privilege, and instead ten of them came from Binyamin. That might then be another reason to call those ten families "tribes."

Answer 2

The reason why Binyamin specifically is referred to as having multiple tribes could be based on a Rashi, that each tribe is also called its own nation is learned from a prophecy concerning Binyamin:

אף חבב עמים. גַּם חִבָּה יְתֵרָה חָבַב אֶת הַשְּׁבָטִים; כָּל אֶחָד וְאֶחָד קָרוּי עַם, שֶׁהֲרֵי בִנְיָמִין לְבַדּוֹ הָיָה עָתִיד לְהִוָּלֵד כְּשֶׁאָמַר הַקָּדוֹשׁ בָּרוּךְ הוּא לְיַעֲקֹב (בראשית ל"ה), "גּוֹי וּקְהַל גּוֹיִם יִהְיֶה מִמֶּךָּ":
אף חבב עמים YEA, HE LOVED THE PEOPLES — Also He loved the tribes with exceeding love. — Each individual tribe may be termed עם (or גוי), “a people”, for, you see, Benjamin alone was yet to be born when the Holy One, blessed be He, said to Jacob, (Genesis 25:11, see Rashi thereon) “A nation and a congregation of nations shall be of you."