Jesus is also a Jew under the law.
Galatians 4
4But when the time had fully come, God sent His Son, born of a woman, born under the law,
In the context of the chapter Jesus is comparing himself to the manna that God sent from heaven. It isn't literally about pre-existing with the manna in heaven.
So Jesus came into existence immediately before Abraham was born?
Correct, but not God.
C.S. Lewis appears to be shamelessly using a false dichotomy, presenting only two options, both of which are extreme, i.e., either Jesus is crazy or he's God. Neither are true. When Jesus was teaching he was teaching the things God was telling him to teach.
John 12
49I have not spoken on My own, but the Father who sent Me has commanded Me what to say and how to say it.
50And I know that His command leads to eternal life. So I speak exactly what the Father has told Me to say.”