In pretty much every other survey platform I've come across, required questions mean that a response must be entered if the question is displayed.Put differently, if a required question is not displayed because of skip logic from an earlier response, that "required" question does not require an answer as the survey logic would override the requirement.
The ODK manual seems to indicate that this is not the case in ODK. Is this correct? Will questions marked as "required" that are skipped due to skip logic result in an upload error? Is it possible to make a response required only if it is displayed?
no it will not result in an error, unless your syntax for the skip logic is incorrect, when the place a require on a field and there is a skip logic, it will skip that field, so that field with the requirement will not appear.
A required question is only strictly 'required' when it is (also) relevant. So any unanswered required questions, which are not currently relevant, will not block an upload. [sorry, lots of negatives in that explanation, but hopefully its still clear: "required" is enforced as "required and relevant"].