How can I provide a respondent with a list of unanswered questions before they submit?

Unfortunately, there is no such way to show only the unanswered questions in the preview. However, you may follow this workaround. You may add a "Preview before Submit button" Here you may add all the labels of mandatory fields so that the mandatory field is not shown while the Form is previewed as the mandatory fields would be forced to fill, the rest fields would be shown on the Form while the form is previewed.

https://widgets.jotform.com/widget/preview_before_submit

Please let us know if you have further questions.

Thanks.