To whom did Moshe speak in the beginning of Matos?

Parshat Matot begins:

ב וַיְדַבֵּר מֹשֶׁה אֶל-רָאשֵׁי הַמַּטּוֹת, לִבְנֵי יִשְׂרָאֵל לֵאמֹר: זֶה הַדָּבָר, אֲשֶׁר צִוָּה ה.

2 And Moses spoke unto the heads of the tribes of the children of Israel, saying: This is the thing which the LORD hath commanded.

and אֶל-רָאשֵׁי הַמַּטּוֹת, לִבְנֵי יִשְׂרָאֵל is a bit unusual. Ignoring the translation above, it can readily be read as being directed to two groups, first to the heads of the tribes and then to the children of Israel. And why specifically here is the target for these halachot the heads of the tribes, rather than the children of Israel?

Rashi writes that there were two separate transmissions, once from Moshe to the roshei hamatos, and once from Moshe to all of Bnei Yisrael. Thus:

the heads of the tribes: He honored the chieftains by teaching them first, and only later the rest of the Israelites. How do we know that he did so with other statements? For it says, “[Moses called to them] and Aaron and all the princes of the community returned to him, and Moses would speak to them. Afterwards, all the children of Israel would draw near” (Exod. 34:31-32). [If so,] why did [Scripture] see fit to mention it here? It is to teach us that annulment of vows may be performed by a single expert, and if no single expert is available, it may be annulled by three laymen. Alternatively, perhaps Moses related this passage to the princes alone? However, here it says, “This is the word,” and in [the chapter dealing with] sacrifices slaughtered outside the Temple confines it [also] says, “This is the word” (Lev. 17:2). Just as there it was said to Aaron, his sons and all the Israelites, as it says, “Speak to Aaron, etc.” (Lev. 17:2), so in this case was it said to all of them. — [Ned. 78a]

ראשי המטות: חלק כבוד לנשיאים ללמדם תחלה ואחר כך לכל בני ישראל. ומנין שאף שאר הדברות כן, תלמוד לומר (שמות לד, לא - לב) וישובו אליו אהרן וכל הנשיאים בעדה וידבר משה אליהם ואחרי כן נגשו כל בני ישראל. ומה ראה לאומרה כאן, למד שהפרת נדרים ביחיד מומחה ואם אין יחיד מומחה מפר בשלשה הדיוטות. או יכול שלא אמר משה פרשה זו אלא לנשיאים בלבד, נאמר כאן זה הדבר, ונאמר בשחוטי חוץ (ויקרא יז, ב) זה הדבר, מה להלן נאמרה לאהרן ולבניו ולכל בני ישראל, שנאמר דבר אל אהרן וגו', אף זו נאמרה לכולן:

Ibn Ezra writes that it was a communication to the heads of the tribes, and then, via them, to the children of Israel. Thus:

וידבר משה אל ראשי המטות ושם כתוב: ככל היוצא מפיו יעשה.

לבני ישראל -
שיאמרו כן לבני ישראל.

Aharon ben Yosef the Karaite is certainly no friend to midrash, and yet he also seems something awkward in this pasuk. He first cites Ibn Ezra as described above, but is unhappy with it. It is more correct that everyone should hear this directly from the prophet himself. Therefore, אֶל-רָאשֵׁי הַמַּטּוֹת לִבְנֵי יִשְׂרָאֵל is written awkwardly, and really should be read with an extra vav. It is el roshei hamatot and livnei Yisrael.

Meanwhile, Rav Saadia Gaon argues with both Ibn Ezra and Rashi, considering it to be "the heads of the tribes of the children of Israel."

Mendelssohn agrees with Rav Saadia Gaon, that it means the roshei hamatot of the bnei Yisrael. He explains the presence of the lamed, which otherwise could be interpreted as there being two targets. Without the heh of the definite article, it could have put everything in the construct form and had roshei matot benei Yisrael. The heh of Hamatot ruined that, and they showed the construct, "of", by means of the lamed.

What is motivating Rashi to say what he said? Well, they are the words of Chazal, from a gemara in Nedarim. It is not always the case that Rashi intends peshat. But here, if this is what Chazal say, and could be read into the pesukim in terms of the order of informing, then he should cite it.

Mendelssohn offers a reason for Rashi beside this, based on the trup. He notes that there is a zakef katon on the word Hamatot and a tipcha on the word Yisrael. And a zakef is a greater pause than the tipcha.

To bring it into Wickes terminology and theory, both zakef and tipcha are pauses of equal value, in that they both subdivide a clause ending in etnacha. But the zakef must have subdivided it first, or else we would not the zakef (for it would be subdividing a clause ending in tipcha). Therefore, we begin with:
וַיְדַבֵּר מֹשֶׁה אֶל-רָאשֵׁי הַמַּטּוֹת לִבְנֵי יִשְׂרָאֵל לֵאמֹר

This is divided into:
וַיְדַבֵּר מֹשֶׁה אֶל-רָאשֵׁי הַמַּטּוֹת
לִבְנֵי יִשְׂרָאֵל לֵאמֹר

which appears to denote two different targets. Otherwise, if we would only have the tipcha dividing first, we would have
וַיְדַבֵּר מֹשֶׁה אֶל-רָאשֵׁי הַמַּטּוֹת לִבְנֵי יִשְׂרָאֵל
לֵאמֹר

and then perhaps
וַיְדַבֵּר מֹשֶׁה
אֶל-רָאשֵׁי הַמַּטּוֹת לִבְנֵי יִשְׂרָאֵל

but dividing livnei yisrael off from el roshei hamatot so early seems to be suggesting that it is not one object.

Perhaps an argument could be made for the trup not intending that; or if it did, being derived from the midrash presented in the gemara rather than being the basis for it. At any rate, it is an interesting connection between peshat and trup.