What you state here implies that Jesus could be argued to be the Mashiach within Judaism, and that some do argue this and subsequently convert. I would put to you that Jesus cannot be argued to be the Mashiach within Judaism, but can only be argued so by one who has already left the doctrine of faith that Judaism espouses.
So, Jews cannot arrive at the conclusion Jesus was the Mashiach, as to arrive at that conclusion one has already had to have left Judaism.
Sure they can. The argument that the Jews had against Jesus is that there are certain prophecies that Jesus didn't fulfill. However, the apostles, who were Jewish, saw that those prophecies would be fulfilled at Christ's return. What happens when Jews turn to Jesus is that they get a full perspective of the Jewish religion. It no longer becomes focuses on the ministry of Moses, but the ministry of Moses focuses on the ministry of Christ.
As Paul stated, a Jew wasn't a Jew by Moses' standards or the Law, but by the promises of Abraham. It was by faith that Abraham was considered righteous and it was by faith that Abraham received the promises of God. So it is also by faith that Moses led the people out of Egypt, looking forward to a land of milk and honey, figurative for the abundant resources of life that are found in God's presence. And so it is by faith that the Jews receive Jesus. They don't forget their customs and traditions, but their customs and traditions are whole for once, or shalom.