Issue #1 - The answer is found in the Great Commission and at the end of John both 20th and 21st chapters and even in the prophets. I will post answers to your other issues in separate posts. Thanks for writing.
The Apostles were uniquely sent, and while the commission is perceived as a general admonition to Christians to spread the gospel, it was uniquely given to them as a mission. They performed that mission. WRITING the gospel down for greater outreach across time and distance is really no different than the Apostles personally telling it.
- Matthew 28:16 Then the eleven disciples went to Galilee, to the mountain where Jesus had told them to go. 17 When they saw him, they worshiped him; but some doubted. 18 Then Jesus came to them and said, "All authority in heaven and on earth has been given to me. 19 Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, 20 and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age."
- John 20:30 Jesus did many other miraculous signs in the presence of his disciples, which are not recorded in this book. 31 But these are written that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name.
- John 21: 24 This is the disciple who testifies to these things and who wrote them down. We know that his testimony is true. 25 Jesus did many other things as well. If every one of them were written down, I suppose that even the whole world would not have room for the books that would be written.
And, of course, there is the prophetic word of Isaiah 55 that testifies to this time after that of the suffering servant of Isaiah 53.
1 "Come, all you who are thirsty, come to the waters; and you who have no money, come, buy and eat! Come, buy wine and milk without money and without cost. 2 Why spend money on what is not bread, and your labor on what does not satisfy? Listen, listen to me, and eat what is good, and your soul will delight in the richest of fare. 3 Give ear and come to me; hear me, that your soul may live. I will make an everlasting covenant with you, my faithful love promised to David. 4 See, I have made him a witness to the peoples, a leader and commander of the peoples. 5 Surely you will summon nations you know not, and nations that do not know you will hasten to you, because of the LORD your God, the Holy One of Israel, for he has endowed you with splendor." 6 Seek the LORD while he may be found; call on him while he is near. 7 Let the wicked forsake his way and the evil man his thoughts. Let him turn to the LORD, and he will have mercy on him, and to our God, for he will freely pardon. 8 "For my thoughts are not your thoughts, neither are your ways my ways," declares the LORD. 9 "As the heavens are higher than the earth, so are my ways higher than your ways and my thoughts than your thoughts. 10 As the rain and the snow come down from heaven, and do not return to it without watering the earth and making it bud and flourish, so that it yields seed for the sower and bread for the eater, 11 so is my word that goes out from my mouth: It will not return to me empty, but will accomplish what I desire and achieve the purpose for which I sent it. 12 You will go out in joy and be led forth in peace; the mountains and hills will burst into song before you, and all the trees of the field will clap their hands. 13 Instead of the thornbush will grow the pine tree, and instead of briers the myrtle will grow. This will be for the LORD's renown, for an everlasting sign, which will not be destroyed."
To God be the glory for preserving His own words:
God's Name is I AM.