[af_vet_1981]

I myself have argued against Paul being the author of Hebrews on the basis of the contrasting style and absence of his otherwise uniform statement of his authorship, yet 2 Peter 3:15 seems to refer to this epistle, and thus I postulate that it may have been originally a synagogue address (to Christians meeting therein) that was penned by someone else, as with the gospels and the teachings of Christ.

I doubt it; Jews who believed in Jesus were generally barred from the synagogues. The epistle was written to these:

• Wherefore, holy brethren, partakers of the heavenly calling, consider the Apostle and High Priest of our profession, Christ Jesus;
  
  ...
  
  
• Know ye that our brother Timothy is set at liberty; with whom, if he come shortly, I will see you.
• Salute all them that have the rule over you, and all the saints. They of Italy salute you.

Hebrews, Catholic chapter three, Protestant verse one,
Hebrews, Catholic chapter thirteen, Protestant verses twenty three to twenty four,
as authorized, but not authored, by King James

But if you want to contend against the Pauline authorship of Hebrews than that is fine with me. However, then you need to thus attribute error to your own council of Trent:

It could have been the Apostle Paul. The part about a synagogue address makes no sense. Had it been to a Greek, Asian, or Roman synagogue, it would have been addressed to a church fathered by the Apostle. It could have been to the churches of Judea and Galilee, and it could have been Pauline, or someone closely associated with the Apostle.

The Pontifical Biblical Communion, in a decree issued on April 24, 1914, reaffirmed its canonicity. It answered the question, “Has the apostle Paul to be regarded as the author of this letter in the sense that not only must one hold that he conceived it and expressed it under the inspiration of the Holy Spirit, but that he gave it the form in which it has come down to us?” Its reply was, “No, not unless the Church decides so in the future.” This is probably why there is no direct reference to Paul as author of this letter in recent liturgical books. However, Paul can be regarded as the indirect author of Hebrews. Researchers are free to explore this matter.

[daniel1212]

I doubt it; Jews who believed in Jesus were generally barred from the synagogues. The epistle was written to these:

Whether Hebrews was possibly originally a address to Christians (not to evangelize but to exhort) meeting in synagogues or houses (believers wherever they could: Justin Martyr says he met above the baths) is not that important. However, while Christians were persecuted by a class of Jews (and locally excommunicated from the synagogue in Jn. 9:22), yet the synagogues were not controlled like as Watchtower houses, or even Catholic churches ("Scholars have noted the lack of evidence for a central organization or leadership structure that oversaw the different synagogues" - https://bible.org/article/origins-church-rome), thus before his conversion, it was in synagogues that Saul found Christians:

And I said, Lord, they know that I imprisoned and beat in every synagogue them that believed on thee: (Acts 22:19)

And I punished them oft in every synagogue, and compelled them to blaspheme; and being exceedingly mad against them, I persecuted them even unto strange cities. (Acts 26:11)

The Jewish encyclopedia informs:

The attachment of the Christian to Jewish customs may be particularly inferred from six sermons, delivered against the Jews in Antioch (about 366-387) by John Chrysostom, later patriarch of Constantinople. On Sabbaths and holidays, Christians, especially women, visited the synagogue in preference to the church. They also preferred to bring their disputes to Jewish judges and took their oaths in the synagogue. - http://www.jewishencyclopedia.com/articles/1586-antioch

Likewise early on many synagogues offered to Paul and others a means of meeting actual evangelization.

And he reasoned in the synagogue every sabbath, and persuaded the Jews and the Greeks." (Acts 18:4)

And he came to Ephesus, and left them there: but he himself entered into the synagogue, and reasoned with the Jews. When they desired him to tarry longer time with them, he consented not; But bade them farewell, saying, I must by all means keep this feast that cometh in Jerusalem: but I will return again unto you, if God will. And he sailed from Ephesus. (Acts 18:19-21)

And he went into the synagogue, and spake boldly for the space of three months, disputing and persuading the things concerning the kingdom of God. (Acts 19:8)

Historian Philip Alexander described a thriving Messianic Jewish community in the Galilee during the second century AD when rabbinic Judaism was emerging. Messianic Jews attended synagogues, lived among other Jews, and practiced Torah. - https://jewsforjesus.org/jewish-resources/community/messianic-jews-a-brief-history/

That would change due to the rejection of Christ and His followers and persecution of them by many Jews, resulting in such antagonism as was expressed by Chrysostom:

How, Chrysostom asked, can Christians “have the slightest converse” with Jews, “the most miserable of all men”? He went on to describe Jews 36 A CONVENIENT HATRED: THE HISTORY OF ANTISEMITISM as “inveterate murderers, destroyers, men possessed by the devil.” To him, the sy nagogue was “a place of shame and ridicule,” “the domicile of the devil.” Indeed, he told his congregation that Jews worshipped the devil with rites that are “criminal and impure” and that the synagogue was “an assembly of criminals,” “a den of thieves,” and “a cavern of devils.”

Why did Chrysostom believe Jews were degenerate? Because of their “odious assassination of Christ.” And for this crime, Chrysostom declared, there was “no expiation possible, no indulgence, no pardon.” In his view, the rejection and dispersal of the Jews was the work of God, not of emperors. He insisted that God had always hated the Jews, and therefore, on Judgment Day, God would say to Judaizers, “Depart from Me, for you have had dealings with murderers.” [John Chrysostom, “Chrysostom’s Homilies Against the Jews.”]

Chrysostom’s attacks had little to do with Jewish practice or belief. He was not interested in real Jews; it was the “Judaizing” Christians he was attacking . In opposing them, however, he demonized “the Jews.” And he was not alone. Other Christian leaders in Chrysostom’s day wrote in similar ways about Jews - Phyllis Goldstein: A Convenient Hatred: The History of Antisemitism; https://www.facinghistory.org/sites/default/files/Ch.2.pdf

It could have been the Apostle Paul.

Then why are we arguing?

The Pontifical Biblical Communion... Paul can be regarded as the indirect author of Hebrews. Researchers are free to explore this matter.

Sounds like what I postulate. But for RCs, the Pontifical Biblical Communion (accused of being liberal by some RCs) versus Trent?