Where does Maimonides write that most halakhot are creative expositions?

In a responsum (I think to Rabbi Pinchas the judge of Alexandria), “except for three or four.”

At first glance:
Rabbi Ishmael before Rabbi Yehuda ben Beteira? The reverse.
The sons of Beteira were the leaders and the bearers of tradition, until Hillel came up from Babylonia…
Obviously, “go out and see what the people do” came before Rabbi Ishmael, who was after Rabbi Yehuda ben Beteira, as Hillel answered them that way.
So we are forced to conclude that this exposition did not generate the prevailing Jewish law, but only attached it textually for the sake of written classification with apparatus.
[Similar to the question to Rav Hai Gaon and his answer regarding teruah, shevarim or teruah—how could there have been uncertainty about it if it was practiced every single year? And his answer there suggests the possibility that different practical traditions were observed simultaneously and tied to different understandings of the verses, until the nation was forced to create a uniform agreed classification in order to preserve unity under difficult non-sovereign conditions and territorial dispersion.]
[General context: Maimonides in the introduction to his Commentary on the Mishnah and in the laws of Rebels: anything that is a law given to Moses at Sinai will not be disputed, and anything that is disputed is not a law given to Moses at Sinai. And see Hershler in Genuzot Geonim regarding Rav Hai Gaon’s responsum on the source of lavud from the Torah, even though it is a law given to Moses at Sinai, and even though there is a dispute whether its measure is three or four handbreadths.]

You should read before you start making proclamations.
I’m talking about the first exposition brought in the question, not the first one chronologically.
And regarding your certainty that this is a merely supportive exposition—that is not really necessary either.
And regarding Maimonides’ puzzling comments about laws given to Moses at Sinai, see the responsum of Chavot Ya’ir 192 at length.

?
“The one who reads the letter should be the messenger…”
Maimonides’ words are not puzzling at all. He used them [rightly] also in The Guide for the Perplexed to strengthen the view of the one who says Job never existed and was not created.
Hershler’s argument [which you didn’t bother to check before sending me to Chavot Ya’ir] is [relying on Rav Hai Gaon] that laws given to Moses at Sinai are accepted principles [like measures, interpositions, and partitions], while their details and measurements were learned through asmachta-style textual supports [as with Rav Hai Gaon and his support for the measure of three in lavud from the curtains of the Tabernacle].
And as for what is necessary and what is not necessary—it depends on the point: whether reading the text is what compels the conclusion, or the desire to innovate against it at any cost [“Rabbi Ishmael said to him: you are saying to Scripture, be silent until I expound it” (Sifra, Leviticus 13:47)] and with no justification except “it isn’t necessary”… because? There you have the proclamation!

His words are very puzzling, and many have already noted this. And the fact that Hershler claims something doesn’t prove anything. He claims that accepted principles are accepted? Fascinating.
I assumed any sensible person understands this and that no explanation is needed. But at your request I’ll add it briefly.
Even if Rabbi Yehuda ben Beteira preceded Rabbi Ishmael, so what? Does that mean the exposition is merely supportive and not the source? Why? I haven’t seen any argument here that requires any justification at all. When there is a later exposition, it could be a proposal to reconstruct the original exposition that generated the Jewish law (as the Netziv writes in Kedmat HaEmek on “they learned it as tradition from the Talmudic text,” according to Maimonides against Rashi), or an asmachta, or an alternative exposition.
You can argue that such a possibility exists (monumental-hyperbolic apparatus-style classification), but that is obviously true. You argued that it is necessary, and that is simply not necessary. And that is exactly what I wrote at the beginning: in most cases there is no way to know. As far as I understand, here too there isn’t.

“Because the Boethusians would say: Shavuot is after the Sabbath.
Rabban Yohanan ben Zakkai confronted them and said: Fools, from where do you derive this? And no one answered him except one old man who kept prattling against him… He said to him: Rabbi, is that how you dismiss me? He said to him: Fool! Should our complete Torah not be like idle chatter of yours? One verse says, ‘You shall count fifty days,’ and another verse says, ‘Seven complete weeks shall there be.’ How so? Here it refers to a holiday that falls on the Sabbath; here to a holiday that falls in the middle of the week.
[And here begins the sequence of bringing the alternative expositions on this issue.]
Rabbi Eliezer [his student] says …
Rabbi Yehoshua [his leading student and the colleague of Rabbi Eliezer, his leading student] says …
Rabbi Ishmael [student of Rabbi Yehoshua, who was a student of Rabban Yohanan ben Zakkai] says …
Rabbi Yehuda ben Beteira says …
Rabbi Yosei bar Yehuda [a tanna from the last generation of tannaim, son of Rabbi Yehuda and grandson of Rabbi Ilai; he lived in the generation of Rabbi Yehuda HaNasi] says …
Rabbi Y. ben Beteira [Rabbi Yehoshua ben Beteira?] says …
Rabbi Yosei [student of Rabbi Akiva] says: Furthermore, it says…
Rabbi Shimon ben Elazar [student of Rabbi Meir, who was a colleague of Rabbi Yosei] says …
[And now it is summarized:]
Rava said: All of them can be refuted except for the two last tannaim, both in the first baraita […] and in the later baraita [Rashi explains: the two tannaim of the later one—namely the “furthermore” of Rabbi Yosei and Rabbi Shimon ben Elazar] which cannot be refuted.
If from Rabban Yohanan ben Zakkai—perhaps as Abaye said…
If from Rabbi Eliezer and Rabbi Yehoshua, who says it refers to the first holiday? Perhaps it refers to the last holiday.
Rabbi Ishmael and Rabbi Yehuda ben Beteira have no refutation.
If from Rabbi Yosei son of Rabbi Yehuda, I might have said perhaps the fifty are in addition to these six.
If from Rabbi Yehuda [?] ben Beteira, who says it refers to the first holiday? Perhaps it refers to the last holiday.
Rabbi Yosei also has a refutation …”
https://he.wikisource.org/wiki/%D7%9E%D7%A0%D7%97%D7%95%D7%AA_%D7%A1%D7%94_%D7%90
Order of presentation:
Rabban Yohanan ben Zakkai [has a refutation]
Rabbi Eliezer [has a refutation]
Rabbi Yehoshua [has a refutation]
Rabbi Ishmael [has no refutation]
[Up to here, chronological presentation]
Rabbi Yehuda ben Beteira [has no refutation]
[Up to here, the first baraita]
Rabbi Yosei bar Yehuda [no necessity]
Rabbi Y. ben Beteira [it doesn’t seem he is Rabbi Yehuda, because then it would have fit to bring his words together and with “furthermore” like Rabbi Yosei; perhaps he is Rabbi Yehoshua ben Beteira, brother of Rabbi Yehuda ben Beteira; see Tosafot s.v. Rabbi Yehuda ben Beteira, who were pressed on this] [no necessity]
Rabbi Yosei [also seems to have a refutation] and furthermore… [has no refutation]
Rabbi Shimon ben Elazar [has no refutation]
[And this is the later baraita]
—
The first baraita: chronological, except for the last one in it, who is actually the first chronologically.
The later baraita: chronological, except for the first one in it, who is actually the last chronologically.
The last one in the first baraita, who is the first chronologically there—and the last one in the later baraita, who is almost the last chronologically there—have no refutation.
That is: both the first and the last end with the correct position, chronologically validated. And both do so by a chronological exception: the first moves the first to the end, and the second moves the last to the beginning. [Pretty logical—this way the order is not changed too much, because the correct validated one is chronologically and in presentation just one place away.]
Rabbi Yehuda ben Beteira and Rabbi Y. ben Beteira [apparently the brothers Rabbi Yehuda and Rabbi Yehoshua sons of Beteira] disagree about the source—Rabbi Yehuda ben Beteira is correct and validated, while Rabbi Y. ben Beteira’s source is not necessary.
Rabbi Yehuda ben Beteira is first chronologically and last in presentation, in the first baraita—and Rabbi Y. ben Beteira is first chronologically, second in presentation, and first among those whose derivations are not necessary, in the later baraita.
The overall chronology apparently is:
Rabbi Yehuda [validated] and Rabbi Y. ben Beteira [not validated]
Rabban Yohanan ben Zakkai [not validated]
Rabbi Eliezer and Rabbi Yehoshua [not validated]
Rabbi Ishmael [validated]
Rabbi Yosei [validated only in his “furthermore”]
Rashba [validated]
Rabbi Yosei bar Yehuda [not validated]
What emerges from this: already from Rabbi Yehuda ben Beteira we had a source validated in practice. But Rabbi Y. ben Beteira, who proposed an alternative source, was not upheld. From there, generation after generation, alternative sources are proposed. But for whom and why? After all, for Rabbi Yehuda ben Beteira, whose source is validated, there is no need for an alternative source. And the practice is already agreed upon by both him and Rabbi Y. ben Beteira, who are chronologically the earliest. [Compare: until Yose ben Yoezer and Yose ben Yohanan there were three disputes, or only one dispute. And from them on…] That is, if there is no dispute in practice, this is a law given to Moses at Sinai [per Maimonides]. Rather, since the sons of Beteira disagreed among themselves about what the source for the practice was—the practice itself being validated!—it necessarily follows that the source was innovated and is not part of the practical Sinaitic law [as Rav Hai Gaon, Maimonides, and Hershler say].

I won’t delete this despite the pedantry and excessive quoting, because there is substantive engagement here. But for our purposes, none of this makes any difference. All of these could be expositions proposed as reconstructions for an existing Jewish law, and it is entirely possible that they were the original basis for it and were what created it in the first place.

“But […] for our purposes [?], none of this makes any difference [why? never mind. The main thing is, blessed is he, ‘that’s just how it is for him’]. All of these could […] be … proposed … and it is entirely possible […] that …”
If the children of your inward parts rejoice over doubts like these, then all the more so over your certainties.
[As you noted, it was a bother for you to read, and for me it was a bother to quote what was needed from the content of the link—so:]
https://he.wikisource.org/wiki/%D7%90%D7%9E%D7%95%D7%A0%D7%AA_%D7%97%D7%9B%D7%9E%D7%99%D7%9D_(%D7%9B%22%D7%99)/%D7%A4%D7%A8%D7%A7_%D7%A9%D7%91%D7%99%D7%A2%D7%99