Paul speaks in metaphors quite often. Calling Christ the second Adam, has nothing to do with the time of His creation. It has to do with God restoring creation. Paul also said that Christ is the first born of all creation. Thus he preceeded Adam.
No, God is referred to as He because He is a person. I didn't call my mother He when she provided, protected, and taught me. I called her, her, because she was a person.
You're correct that the Father isn't referred to as the Father in the OT. That doesn't mean He wasn't one. There are references to the Son in the OT.
Psalm 2:1–7 (KJV 1900): Why do the heathen rage,
And the people imagine a vain thing?
2 The kings of the earth set themselves,
And the rulers take counsel together,
Against the Lord, and against his anointed, saying,
3 Let us break their bands asunder,
And cast away their cords from us.
4 He that sitteth in the heavens shall laugh:
The Lord shall have them in derision.
5 Then shall he speak unto them in his wrath,
And vex them in his sore displeasure.
6 Yet have I set my king
Upon my holy hill of Zion.
7 I will declare the decree:
The Lord hath said unto me,
Thou art my Son; This day have I begotten thee.
Proverbs 30:4Who hath ascended up into heaven, or descended? Who hath gathered the wind in his fists? Who hath bound the waters in a garment? Who hath established all the ends of the earth? What is his name, and
what is his son’s name, if thou canst tell?
So, even though He's not called Fathee in the OT, we see that He was one
Incomprehensible means something can't be comprehended. Maybe you're looking for a different word.