If I turn off the Comment feature during an active survey, will users who haven't taken the survey yet be able to leave comments?

No, the user will no longer be able to leave comments on the survey question but will still be able to answer any open ended questions in the survey. These answers will be saved as usual, but they will not be shown in the tool as the feature is no longer active. As soon as the feature is enabled you will be able to access the answers in the comments section.