The default should be to show one comment at a time, because that's convenient and quick to get into, but also with an option (maybe just a scroll down) to view all comments. One, that helps the reviewer get an overall idea of what kind of things the submitter is looking for, if they want that, and two, some submitters are inevitably gonna screw up, posting at the wrong times or asking overall summary questions that should be asked at the end right at the beginning or somesuch. So an All Questions button or similar should be there as an escape hatch, but not the primary UI.