How about Barnabas? Why was his Epistle rejected from the Canon? Many argues for its inclusion and before the Canon was set many considered it Scripture. Barnabas was closely related to several Apostles. Barnabas is even named several times in the Acts of the Apostles. The entire Epistle of Barnabas with its 21 chapters was preserved complete in the 4th century Codex Sinaiticus where it appears at the end of the New Testament.
Correct. There were many accounts written over the years, and many versions of each book written according to the copyist.
The answer to why the NT was chosen rests entirely with the Council of Nicea and the Holy Spirit. We don’t know exactly why the Didache or Hermas were rejected and John’s Apocalypse included. Same with rejecting Barnabas and including Hebrews.
And the Jews were big on pseudoepigraphic works, so we don’t really know who wrote what for the most part. The point is that the Church has canonized Scripture and therefore it is canonized.