I agree with nowwhat. If the elders did things according to the Society's rules, they will already have determined guilt or innocence before any JC is formed. So that the JC itself would be more akin to a sentencing hearing, where the elders are trying to determine if there is any reason for leniency. (Of course, there is an outside chance that new evidence presented during a JC might change presumed guilt to innocence. But that would be the exception rather than the rule. Any new evidence would probably be met with the elders wondering why you didn't present this evidence earlier.)
If the elders had determined that a person wasn't guilty of the alleged sin, then, there would be no JC formed. Any non-JW witnesses to the alleged 'crime' would have been heard from before the JC was formed.
And that's probably why non-JWs would most likely not be allowed into a JC. At that point they would only be seen as character witnesses. And the elders would have no use for such.
Of course, all that is assuming they were going by WT rules. That doesn't always happen. The WT and elders would probably not admit to this, but KS schools spend a lot of time dealing with JC issues where the elders didn't follow their rules.