Good questions. I'll try my best to answer them all without sounding "preachy". Here goes....
1) Following your supposition, I would have no basis for my faith in Jesus. For the true Christian, our faith is based on Christ's atoning sacrifice for our sins, for the debt He paid in our place in order to reconcile us to God the Father. If Jesus had been just an ordinary man, sinful in nature like the rest of us, then I would remain guilty because a mere man could not have atoned for the sins of the world. Also, because under OT law God established a blood covenant relationship between Himself and us, Christians would still have to continue to offer animals to high priests to sacrifice for our sins. The reason we no longer adhere to this OT law is because Christ fulfilled the law as the final and perfect sacrifice.
For the record though, I believe Jesus was both fully God and fully man, that He is God incarnate (God in the flesh), 2nd in the Trinity (One God but triune in nature). Scripture supports this, but I will respect your wishes and not post them to try and convince you otherwise.
2) This is an excellent question! Even though there is overwhelming evidence that the New Testament is an accurate and trustworthy historical document, there should also be some independent, non-biblical accounts that corroborate Jesus's existence. And there is! From both hostile and neutral sources. To keep from copying and pasting all that information, here is a link for you to pursue at your leisure: http://www.probe.org/content/view/18/77/
Also, I would recommend a book called, "The Case for Christ: A Journalist's Personal Investigation of the Evidence for Jesus" by Lee Strobel. Lee Strobel attended Yale Law School, is an award-winning former legal editor of the Chicago Tribune, and was an atheist before this investigation. Summaries and reviews can be found here: http://www.amazon.com/Case-Christ-Journalists-Personal-Investigation/dp/0310209307/sr=1-1/qid=1170663044/ref=pd_bbs_sr_1/002-8446737-0096055?ie=UTF8&s=books
3) As part of the triune God (God the Father, God the Son, and God the Holy Spirit), Jesus was divine in nature from the beginning. He didn't start out as merely human and then become divine. I'll admit, the Trinity is a difficult concept for our finite minds to grasp. It would be kind of like trying to explain ourselves to an ant. The best analogy I've read is water (not that God can be summed up by this analogy, mind you). It has three properties: liquid (water), solid (ice), and gas (steam). One substance; three different properties - One God, three different distinctions. Someone else might be able to explain this concept better than I can though.
For the second part of your question, all believers in Christ are united with God in spirit (1 Corinthians 6:17) and in purpose. Once you accept God's gift of salvation, you undergo a spiritual rebirth (John 3:3) and are sealed with the Holy Spirit (Ephesians 1:13-14), who lives inside each believer.
4) John (sinful man) did not feel worthy enough to baptize Jesus (sinless Lord), and who could blame him, so Jesus had to convince him by telling him that it was proper in order to fulfill all righteousness. This is the only instance I can think of where skeptics might be able to say that Jesus "purposely" fulfilled prophecy.
Other fulfillments such as the circumstances regarding His birth, His miracles, the details of His death - right down to the Roman soldiers casting lots for His clothing, and His resurrection are fulfillments of prophecy that could not have been manipulated by some ordinary man who falsely claimed to be the Messiah. Jesus was the real deal.
Also, the Dead Sea Scrolls, which comprise the oldest group of Old Testament manuscripts ever found, date back to 100--200 B.C. This is dramatic, because we now have absolute evidence that Messianic prophecies contained in today’s Old Testament (both Jewish and Christian) are the same Messianic prophecies that existed prior to the time Jesus walked on this earth.
And Jesus fulfilled those prophecies, thus proving He is the promised Messiah!
I hope this was helpful!