Finally, why didn't any of the other unorthodox accounts start similar communities? Why are the communities so similar in their beliefs about Jesus, if it is quite likely to have been made up, as you suggest?

They did. There were plenty of wildly disparate sects of Christianity early on, out of whose beliefs the gospels that were declared noncanonical rose up in the first place. Most of these communities died out over time, although some lasted for centuries, and Gnostics, who existed in several branches and were the most significant competitors for what became "mainstream" Christianity, still exist today. I suggest reading up a bit on Gnostic mythology to see just how disimilar it is to all mainstream branches of Christianity.

As for why the apostles would die martyrs' deaths for someone who didn't live up to his promises, we have no reason to think they did. That is, they weren't killed for their beliefs after being given the option of recanting to have their lives spared. This is a popular meme, but there's no historical basis for ever suspecting it happened in the first place. jhwendy's link at the bottom of the page provides more information on this.

As for the Christian communities being able to filter true and false accounts of his life, we already know that they didn't; witness the proliferation of sects and gospels declared noncanonical. Most of them had probably never known him during his life.