I haven't read this entire thread... But I wanted to put in my two cents real quick...
Providing the pre-existance of Jesus is quite simple, and is plainly taught in scripture. Any attempt to deny this would be by twisting scripture.
However, just because we have very clear biblical evidence of pre-existance doesn't in anyway support trinitariansm. Just providing pre-existance, means He existed in some way, shape or form before He became human.
Many religions and denominations accept the biblical concept of the pre-existance of Jesus... If Jesus did exist, prior to His human appearance, 2000 years ago, then who is He?
Some believe He is the Spirit Brother of Satan...
Some believe He is the Arch Angel Michael.
Some believe He is a second person of a triune god.
Some believe He is God Himself manifested in the flesh.
Providing pre-existance, only gets you to the obvious. If you are trying to prove a trinity, you have to provide biblical evidence, that not only He existed, but He existed alongside with two others, each being 100% God, co-existant, co-eternal, and co-equal. You also have to provide how this is in someway not polytheistic. You can say "we believe in One God" all day long, but when you begin to describe the trinity you begin to describe 3 gods.