They Shall All Be Taught By God

Which Old Testament passage is Jesus paraphrasing in John 6:45?
“And all your children shall be taught by the LORD, and great shall be the peace of your children” (Isaiah 54:13).

Who does Jesus say taught the people who come to believe in Him?
God Himself: “They shall all be taught by God” (John 6:45).

Is that true?
God may teach people directly, which is what Jesus did to His disciples, or He may use human vessels to teach. In both cases, it is God Himself who opens our mind to understand His truth (see Spirit of Truth). If God doesn't grant this understanding, you may hear, read, and even gain knowledge of the Bible, but you will not have truly “heard from the Father and learned” (John 6:45) His spiritual truth that draws you to Jesus for salvation (see No one can come to Me unless the Father who sent Me draws him).

Why does Jesus say, “It is written in the prophets” (John 6:45) instead of saying it is written by the prophets, or it is said by the prophets?
The Hebrew Tanakh, which has the same content as the Bible's Old Testament, is organized into three parts, and Isaiah is in the second part, called Nevi'im, which means Prophets (see Tanakh vs. Old Testament).

Who is the only “One” (John 6:46) who has seen God the Father?
Jesus.

Is it true that nobody but Jesus has seen God the Father?
εωρακεν (heoraken), the original Greek word twice used and translated “has seen” in John 6:46, means seeing with the mind - i.e., 'understanding' - as well as with the eyes. We use the word “see” in the same sense when we tell someone, “I see what you're saying”; we're not saying that we can visualize someone's spoken words, but rather that we understand what they are saying. There is a limit to how much we creatures can understand our Creator. Jesus is saying that nobody but He truly understands God the Father.