Answers to 2-4: Because we interpret the Scriptures through the lens of Tradition, which gave us these Scriptures.
Just like you accept Tradition each and every time you quote, say, 3 John, as being inspired.
You don’t have to trust what I say. You have already trusted what the Catholic Church says, at least when it comes to defining what’s inspired and what’s not.
You trust that the Church got it right in keeping Revelation in the canon, and excluding the Didache.
You have no other reason to believe that Revelation is inspired…
except that the Catholic Church, under the guidance of the Holy Spirit, told you it was.