sorry for the delay in answering, it's been some crazy months lately.
We don't have any specific plan for quiz essay attachments, I think it needs WS development and AFAIK it's not planned to be included in Moodle 3.6, so it can still take a while to be supported.
Offline quiz and time limits won't ever be supported (unless the quiz code is refactored, and I don't think that's gonna happen). The reason is that when the user downloads the quiz an attempt is automatically started, so the timer would start counting even if the student hasn't opened the quiz.
Most of the activities have been simplified for the mobile app (e.g. in lesson or wiki you only have access to some of the features), it would take too long to list all of the things that are missing in the app. We plan to add some of them in the mobile app, like allowing a teacher to see list of quiz submissions and giving feedback, but some other features aren't really suited for the mobile app.