The resolution to the above questions can be understood by prefacing with an explanation of the difference between the mitzvos of sukkah and the four species.וְיוּבַן זֶה בְּהַקְדִּים הַחִלּוּק שֶׁבֵּין סֻכָּה לְד' מִינִים.
Although both these mitzvos reflect the concept of unity, as will be explained (below),דְּעִם הֱיוֹת שֶׁשְּׁנֵיהֶם מוֹרִים עַל עִנְיַן הָאַחְדּוּת (כְּדִלְקַמָּן),
nevertheless, there is a difference between them.אַף עַל פִּי כֵן חִלּוּק בֵּינֵיהֶם.
The oneness expressed by the sukkah is essential unity,שֶׁהָאַחְדּוּת דְּסֻכָּה הִיא אַחְדּוּת בְּעֶצֶם,
a unity that, (from the outset,) does not recognize any distinctions.שֶׁאֵין בָּהּ (מִלְּכַתְּחִלָּה) הִתְחַלְּקוּת פְּרָטִים.
As explained in Chassidus,1 the mitzvah of sukkah elicits an indivisible encompassing light (or makkif), which transcends all distinctions. Distinctions between different entities originate in those levels of G‑dliness that relate to the created realms. The sukkah, however, relates to an essential dimension of G‑dliness that is entirely above the framework of creation, and which hence knows neither division nor distinction.
This essential, transcendent dimension is mirrored within the Jewish people.
This is the reason that “All the Jewish people are worthy of dwelling in one sukkah,”2שֶׁלָּכֵן כָּל יִשְׂרָאֵל רְאוּיִים לֵישֵׁב בְּסֻכָּה אַחַת,
for a sukkah is above all distinctions. Hence, it can encompass the entire Jewish people. Moreover, it brings out the essential dimension possessed by all Jews, which eclipses the particular differences that exist between them.לְפִי שֶׁסֻּכָּה הִיא לְמַעְלָה מֵהִתְחַלְּקוּת,
In contrast, the mitzvah of taking the four species reflects a different motif:מַה שֶּׁאֵין כֵּן עִנְיַן מִצְוַת נְטִילַת ד' מִינִים
the transformation of multiplicity into a unified whole.הוּא שֶׁמֵּרִבּוּי נַעֲשָׂה אַחְדּוּת.
Although they – the Jewish people – can be compared to four diverse “species” radically differing from one other, they nevertheless merge together as one single unit.דַּהֲגַם שֶׁהֵם ד' מִינִים שׁוֹנִים וּמִן הַקָּצֶה אֶל הַקָּצֶה,
As the Midrash relates,3 the four species comprising the mitzvah of lulav correspond to four different types of Jews,כִּדְאִיתָא בַּמִּדְרָשׁ דְּד' מִינִים שֶׁבַּלּוּלָב הֵם כְּנֶגֶד ד' סוּגִים שֶׁבְּיִשְׂרָאֵל,
ranging from the type of Jew personified by the esrog, which possesses both pleasant flavor and fragrance (metaphorically alluding to the study of Torah and the observance of mitzvos),מֵהַסּוּג שֶׁכְּנֶגֶד אֶתְרוֹג שֶׁיֵּשׁ בּוֹ טַעַם וָרֵיחַ (תּוֹרָה וּמַעֲשִׂים טוֹבִים)
to the type symbolized by the willow which has neither flavor nor fragrance.עַד לְהַסּוּג שֶׁכְּנֶגֶד עֲרָבָה שֶׁאֵין בָּהּ לֹא טַעַם וְלֹא רֵיחַ,
Nevertheless, all four types unite to form one unit.וְהֵם נַעֲשִׂים כֻּלָּם אֲגֻדָּה אֶחָת.
In Chassidus,4 it is explained that the unity highlighted by the four species is mirrored within each one of the species themselves. In that context, as well, the unity reflected in each species indicates how unity results from multiplicity. For example, the fact that the esrog grows throughout all the four seasons demonstrates how it unifies every type of influence. This reflects how unity results from multiplicity. Similar concepts apply regarding the other three species.
This concept also applies to the Midrashic interpretation5 that each of the four species alludes to the Holy One, blessed be He.וְגַם לְהַפֵּרוּשׁ שֶׁד' הַמִּינִים רוֹמְזִים עַל הקב"ה,
From the fact that spiritual qualities (attributes of the Holy One, blessed be He) are alluded to by the four species,הִנֵּה מִזֶּה שֶׁהָעִנְיָנִים דִּלְמַעְלָה (הקב"ה) מְרֻמָּזִים בְּד' מִינִים,
it is understood that they reflect four different levels of G‑dliness.מוּבָן, שֶׁהֵם ד' מַדְרֵגוֹת שׁוֹנוֹת
[In accordance with the well-known concept that all interpretations of one verse are interrelated,6[וְעַל פִּי הַיָּדוּעַ שֶׁכָּל הַפֵּרוּשִׁים שֶׁבְּאוֹתוֹ הַפָּסוּק שַׁיָּכִים זֶה לָזֶה,
it can be posited that the differences between the four levels of G‑dliness alluded to by the four species are radical, ranging from one extreme to the other,יֵשׁ לוֹמַר, שֶׁהַחִלּוּק בְּד' הַמַּדְרֵגוֹת דִּלְמַעְלָה הַמְּרֻמָּזִים בְּד' הַמִּינִים הוּא מִן הַקָּצֶה אֶל הַקָּצֶה,
comparable to the differences between the four above-mentioned types of Jews alluded to by these species.]בְּדֻגְמַת הַחִלּוּק שֶׁבֵּין ד' סוּגִים הַנִּזְכָּרִים לְעֵיל דְּיִשְׂרָאֵל],
Through fulfilling the mitzvah of taking the four speciesוְעַל יְדֵי מִצְוַת נְטִילַת ד' מִינִים
[for taking all four species comprises a single mitzvah],7[שֶׁכָּל ד' הַמִּינִים הֵם מִצְוָה אַחַת]
these different dimensions of G‑dliness become united. Thus, on this level as well, the mitzvah of the four species reflects a unity in the framework of multiplicity.הֵם מִתְאַחֲדִים.

Summary

Both the mitzvah of sukkah and that of the four species reflect the concept of unity, but they highlight two different dimensions of unity. The sukkah reflects a level of oneness that transcends all particular distinctions, while the four species highlight the establishment of unity amidst multiplicity.