Apple's reviewers don't have access to the original CSAM to know if it's a match or not. That stays with NCMEC. If they see some legal porn that looks like it could be illegal, they'd likely flag it as a match.

Roughly thirty images is the threshold for the system to activate, but would they need to review all thirty images to pass it on or would they just need to verify one image looks visually like CSAM in order to pass it along?

It seems unlikely in the event that there was anything that they verified as CSAM they wouldn't pass it on just because they found a false positive in those thumbnails.