So those there persons (Father, Son and Holy spririt) are not beings?
No. From Lateran Council IV: DS 800.
We firmly believe and confess without reservation that there is only one true God, eternal infinite (immensus) and unchangeable, incomprehensible, almighty and ineffable, the Father and the Son and the Holy Spirit; three persons indeed, but one essence, substance or nature entirely simple.
Does that mean that God is merely scizophrenic?
No
And how does that mesh with the idea that God is “simple”?
What do you mean by the word simple? You seem to imply that simple equals easy to understand.
Was Jesus not a “being” just a “person”? Was he not composed of the same complex material as other humans are? Did he not have a nervous system which allowed his suffering?
Jesus is the hypstatic union of God and man. At once divine and human. One person, two natures. His body was made up of the same types of material yours and mine are. The book of Hebrews records:but one who in every respect has been tempted as we are, yet** without** sin. Heb 4:15 RSV
Was his suffering a pretense or an illusion?
Neither. It was real.
Did Jesus not go through a childhood and adulthood, which clearly implies change as opposed to God being unchanging and immutable?
Addressed in the CCC article 470Because “human nature was assumed, not absorbed”,97 in the mysterious union of the Incarnation, the Church was led over the course of centuries to confess the full reality of Christ’s human soul, with its operations of intellect and will, and of his human body. In parallel fashion, she had to recall on each occasion that Christ’s human nature belongs, as his own, to the divine person of the Son of God, who assumed it. Everything that Christ is and does in this nature derives from “one of the Trinity”. The Son of God therefore communicates to his humanity his own personal mode of existence in the Trinity. In his soul as in his body, Christ thus expresses humanly the divine ways of the Trinity:98
The Son of God. . . worked with human hands; he thought with a human mind. He acted with a human will, and with a human heart he loved. Born of the Virgin Mary, he has truly been made one of us, like to us in all things except sin.99