1. do they need to reject the example and teachings of Jesus of the Gospels, why would they not believe or support any spiritual principle Jesus taught?
1a. Because nothing he added is useful. We have sources of actual authority to rely on. Why listen to some random itinerant teacher who was taking from another source that we already have? And when he changed things, why follow a person who is not in a position to change things?
2. Next question (perhaps to Jews), who was that "Messiah" the Jews believe in and whom will fill those shoes besides Jesus? Lol I mean it's pretty obvious Jesus was the promised DELIVERER, that's what Messiah means. The name of Jesus is hardly anything to not recognize as a Messiah.
2a. Messiah (in English) comes from "moshiach" in Hebrew which means "one who is anointed", not "deliverer". High priests and kings were anointed with a particular oil. Jesus wasn't, nor was he eligible to be either a king or a high priest. We await a king from the proper lineage and of the proper personal characteristics.
3. Jesus' body WAS purely human, however His soul and message was that of God's own. Jesus actually hardly ever went around proclaiming He was God, rather always spoke of the Father, and when He was questioned He said Him and the Father were one, that's true if you understand unity on a spiritual level.
3a. You can try to make the argument that the body was human but the soul wasn't, but Judaism's conception of the messiah is that he is a full on human in body and soul.
4. Really lol? you mean He may not have fulfilled a few misconceptions?
4a. Are you saying that Jews don't understand their own texts? There are some very clear expectations and he didn't meet them.
5. Ouch, that's entirely not true and if it were, we wouldn't have anything to consider in this topic. Jesus would have never been acclaimed Messiah.
5a. What Jesus might have "fulfilled" were of 3 categories -- the first were prophecies that were so general that many people fulfilled them (where someone is born - lots of people are born in lots of places), the second are intentional (mis)appropriations of biblical text said to apply to him (wait, Isaiah said that and Jesus said it?! WOW!) including some mis-statements and misinterpretations (the Hebrew word for virgin is Betula...) and the third are textual citations which simply weren't ever biblical prophecies but which, it was decided by non-Jews, were "shadows of things to come" (Jews sacrificed a lamb, Jesus was called a "lamb" so the biblical sacrifices must have been a hint to Jesus!). Choplogic, all of them.
6. Deliverer not supporter.
6a. This is tougher because Jesus does make claims about not coming to end Jewish law and yet people still say that he delivered people from the yoke of the laws. They want to see him as supporter of the Jewish legal system and yet a rebel against it.
7. Not that the concept of the Trinity is not Christian doctrine, but can you show where Jesus uses that term? if Jesus did not use that term perhaps He meant something different....however, the so-called Trinity also lies in the understanding of unity. Unity binds things together as one unit and one purpose.