Again, it's not a matter of "proof". It's a matter of reasonable doubt.
Same thing with happens in the court system when a prosecutor tries to establish someone's motive for doing something. He or she can't "prove" that the defendant was thinking or feeling X, but they can show all the reasons why the defendant might have thought or felt X. It's then up to the jury to determine whether the argument is reasonable or not. You're not going to be able to establish "proof" in the context you're using it.
The problem that you're going to have here is that christians have a vested interest in not believing that the jesus myth is not unique (see: willful ignorance). Therefore you're going to run head-first into a biased audience that has a clear motivation not to be objective.
The best you can do is study the history anyway and present your arguments as you care to. Do so knowing that while you won't be able to reach "true believers", your case might be persuasive to those that have doubts, are persuaded by reason, or haven't committed themselves to religious dogmatism.