Questions 3 and 4 are definitely related, from a historical perspective.
The formation of a distinctly Christian canon likely began as a reaction to the spread of Marcionism, as Marcion was an early heretic of note who did not believe that the God of the OT was the Father of Jesus Christ, and hence separated the OT from the NT writings, and discarded the OT. He also produced an edited collection of NT writings (including some that his followers attributed to St. Paul, but are not found among St. Paul's accepted writings), which were made to conform with his theological views. He was thrown out of the Church c. 140s AD, so this was quite early on if we consider that the last of the gospel writers to die, St. John, did not do so until around the turn of the first century.
For some time after this the individual writings circulated, but were not 'canonized' like we would think of them today, with instead some 'regional canons' of sorts evolving in different places (see below). There was a large variety of both OT and NT apocrypha, and some of the books that we do not think of as being part of the 'standard' OT or NT today (like the Book of Enoch, or the Shepherd of Hermas) actually had considerable supporters in earlier. We recently had a thread here on CF (though I can't remember exactly where or what it was called, or else I'd link it here) asking if Jesus Himself quoted from the Book of Enoch, and some very respected and orthodox sources like HH St. Athanasius of Alexandria are known to have referenced the Shepherd of Hermas.
It is in this climate that several peoples who have been Christian since before even the acceptance of Christianity by the Roman Empire (380 AD) or the earliest extant 27-book NT list (367 AD; in the 39th festal letter of HH St. Athanasius), like the Ethiopians and the Syrians, first received the scriptures. Hence these peoples have some differences from the canon because they received the books themselves in this 'pre-canonized' state: the books were all around, they were available to be read, but nobody had come out and said "we should have only these, and not these others". Such language
is in HH St. Athanasius' letter (where he says, for instance, that there is no room for 'secret' readings -- thereby eliminating any gnostic gospels, of which there were many in his native Egypt), but the Syrians and Ethiopians were already reading the scriptures by then. And some of the books that the Ethiopians retained, presumably from Greek or Hebrew (or sometimes Syriac or Coptic) originals, later became unavailable in any but their Ge'ez/Classical Ethiopic recensions, and are therefore now known as distinctly 'Ethiopian' books, as they are distinctly found in the Ethiopian/Eritrean biblical canons (this is the case with the Book of Enoch, for instance; the only reason we even know of it in English in our own day is because of relatively recent translations made from medieval Ethiopian manuscripts).
In the case of the Syrians/Syriac-speaking people, their Pešitto version (Syriac Bible) was translated from Hebrew for the OT text by the 2nd century AD, and from Greek for the NT text by the 5th century AD (there were earlier Syriac versions of the NT before this, but the Pešitto is the standard; see
this article on the Old Syriac Version, which is somewhat misleadingly titled, as there was more than just one). Relevant to this discussion is the fact that the Book of Revelation, 2 Peter, 2-3 John, and Jude were not added to the Syriac churches' canon until the so-called 'Heraklian Version' of HG Thomas of Herqel in 616 AD.
By these two examples, I hope you can see that there wasn't any one person who gave the stamp of authenticity to this or that Biblical canon. Churches have always had some differences in Biblical canon, depending on the circumstances under which they first received the scriptures.
Though it should be said that in at least one ancient case, we can more or less thank one man for the existence of the scriptures, at least in a very specific sense: The Armenians had converted as a kingdom a century before they even had a written language, so they originally worshipped in Syriac until the invention of the Armenian alphabet by St. Mesrop Mashtots c. 405 AD. So we can assume that their earliest versions of the Bible may have been 'missing' several books relative to the canon as we know it today, since this was well before the Syriac Bible itself would've had the books described above as part of their regular canon. (St. Mesrop was a very smart man, however: in collecting materials to produce his version of the Bible in written Armenian, he did not just rely on the preexisting Syriac scriptures that were already known in Armenia, but also sent his students around the Roman empire to collect the best manuscripts they could from the preexisting centers of Christianity like Constantinople, Antioch, Alexandria, and Edessa. His life is fascinating, and if you are interested in knowing more about him and the establishment of Christianity in Armenia, I highly recommend
'The Life of Mashtots' by his disciple Koriun.)
Such differences are not necessarily considered a problem for the churches and peoples concerned. The Ethiopians and Eritreans have a much larger canon than any other church in Christianity, but they are by their history rightly considered daughters of the Coptic Orthodox (Egyptian) Church, since it was from Alexandria that they received their first bishops after the death of the founder of their Church, St. Frumentius (d. 383), who was a Syro-Phonecian Greek from Tyre (Lebanon) who converted the Axumite king 'Ezana to Christianity c. 330 AD.
I hope these points give you a little bit more to think about, OP.
