So what if it wasn't a physical and incarnate God in the OT? Is that the only way you believe God can or chose to communicate with us? If so, then you do not believe in an indwelling Holy Spirit.
The beauty of the Gospels as a witness is that more than one person saw and heard Him.
You yourself have pointed out that there are several passages in the Gospels in which there could have been no other direct witnesses, such as His trials in the desert and when He prayed to take the cup away. Yet, (I hope :) you accept those as true. The scriptures are authenticated by GOD, not by how many humans were around to vouch for them.
Yes the Incarnation is the ultimate revelation of God as He wanted us to perceive Him (with our limited senses). That is the "core" belief of Christianity, FK. God is ineffable, invisible Spirit who is nothing like us and whom we can neither know nor relate to. Christ, on the other hand, is both visible and comprehensible, a Person we can relate to, imitate and follow (in His humanity). That's why we can only come to God through Him, through His humanity.
Indwelling Spirit is, I believe, something St. Paul introduced. If He means that the love of God is in our hearts and minds, and that we imitate God in our spirit, I can agree with that concept but I think your side has something else in mind, more like an alien presence that was implanted in the "elect."
You yourself have pointed out that there are several passages in the Gospels in which there could have been no other direct witnesses, such as His trials in the desert and when He prayed to take the cup away. Yet, (I hope :) you accept those as true
I believe in the message of the Gospels, as I do in the message of the whole Bible when the message is Christ-like. As far as His trials in the desert is concerned, the Gospels, if I remember correctly, do not agree as to just when did He go to the desert (i.e. right after the Baptism, which is in an of itself strange), or after several days. So, there is some room for doubt.
Clearly, the Gospels could not be eyewitnesses to everything there is in them. They are narratives which in most places corroborate themselves. In other places, the accounts differ or are even not mentioned (for instance John doesn't even talk about the trial of Jesus by the Sanhedrin! In other places the same conversation takes on a life of its own, such as the alleged exchange between Christ and Pontius Pilate, what Jesus said on the Cross, or how many women discovered the empty tomb, how many angels were there, etc., etc., etc.)
I think you are concentrating on the the stories more than on the message behind them. The essence of Christ's teachings is fundamentally different from the angry messages of other Jewish "messiahs," including St. John the Forerunner/Baptist. He was as unlike the Jewish warrior-king as it gets.
The second problem is that at least two our of four Gospel writers are not eyewitnesses (Mark and Luke), and at most that all four were not the Apostoles to whom the Gospels are attributed. However, the message is a compilation of what is probably part legend and part eyewitness account of Jesus' ministry which the mainline Christians accepted and lived by in the 1st century A.D.
The scriptures are authenticated by GOD, , not by how many humans were around to vouch for them.
Every other religion says that for their holy books. That's about as convincing as saying "it must be true, it's in the New York Times," FK. You must already believe that in oder for that to be true, so no matter how you look at it, a believing human is always vouching for them being true.