The focus will be on deferred feedback quizzes. That is, the focus is on allowing the students to continue to input their responses to all questions in a multi-page quiz when offline, and later have the reposes sent back to the server. The issue is not about generating immediate feedback on the client side.
The idea is that, as well as storing the student's response to every question on the current page in client storage, we also store the rendered HTML of the body of each page. Then, students can switch between pages (having any more recent attempt data pulled out of the client-side auto-save) and input all their responses.
Later, when the internet connection is back, we can send al the responses to the server.
As well as robustness (assuming that we can make this work reliably) this would also have the benefit that switching between pages of the quiz is instant.
Because this may be used for exams, we will provide an option to encrypt that download. (Probably some sort of public-key encryption, so the file can only be uploaded to the original Moodle site.)
We will also need a script (quiz report?) which can be used to upload these saved responses.