Regarding John 8:2-11, I could not find one translation that omits that passage though some do include a footnote saying, “early manuscripts omit these verses” and some use “brackets” at the beginning and end which suggest they were not in the original autographs of Scripture. And then there are those who show “internal evidence” that John 7:53-8:11 is not original to the text. They basically say that this passage disrupts the context and then they quote verses 7:52 and 8:12, “They (the Pharisees) answered and said to Him (Jesus), ‘Are you also from Galilee? Search and look, for not prophet has arisen out of Galilee.’ The Pharisees therefore said to Him, ‘You bear witness of Yourself; Your witness is not true.” One must admit “this makes sense” and the passage of the woman taken in adultery “seems out of place.” Having said this, many translations are based on the well-known and highly respected “Textus Receptus” manuscripts and they include John 8:2-11. And there are many other Greek manuscripts that also included the story of the woman taken in adultery. 

If we were able to find the “original autographs” of all the INSPIRED WRITERS of the Bible, this case would be resolved, for those were written directly under the guidance and control of the Holy Spirit. We read of this in 2 Peter 1:21 that, “holy men of God spoke as they were moved by the Holy Spirit.” We can say with full assurance that every word in those autographs is inerrant (without error). But God has chosen to keep these from us for reasons unknown to us now. At the same time, we can be assured that whatever mistakes (copyist errors of some kind) were made, the TRUTH OF GOD was not compromised in any way, shape, matter, or form. God has sovereignly PRESERVED all vital and foundational truths of Scripture…. the truth of creation, the Trinity, the fall of man, heaven and hell, the Person and Work of Christ, salvation through simple faith in Christ and His finished work on the cross, the Second Coming of Christ, and many other truths.

As to your second question, we have seen that “holy men of God spoke as they were moved by the Holy Spirit” (2 Peter 1:21), which means the disciples of Jesus did not have to be with Him to write of what He did when He was alone. I am sure no true believer in God and His Word would doubt the account of creation given by the inspired writer of Genesis chapter one. Was he there when “God created the heaven and the earth?” Absolutely not! Yet the Holy Spirit guided his pen when he wrote those sacred scriptures revealing to us that a personal and sovereign God, who is eternal and all-powerful, created the heaven and the earth and everything in them by simply “speaking a word.” The inspired Psalmist wrote, “By the word of the LORD the heavens were made, and all the host of them by the breath of His mouth…For He spoke, and it was done; He commanded, and it stood fast” (Psalm 33:5, 9). The writer of Hebrews echoed this same truth, “By faith we understand that THE WORLDS WERE FRAMED BY THE WORD OF GOD, so that the things which are seen were not made of things which are visible” (Hebrews 11:3). These three “inspired men” were all absent when God “spoke, and it was done,” yet God the Holy Spirit was there, and He guided all three men to give us this marvelous revelation!  (DO)  (660.3)