Is it possible to Export everything to Google Tasks?
If not, how does it work if someone has to temporarily move off Todoist to Google Tasks?
Unfortunately, that's not possible. We always encourage our users to use Todoist as an alternative to Google tasks, not a complimentary tool as we also offer a variety of browser plugins so Todoist can be much better than Google Tasks, you can access it from anywhere and also add your Gmail emails to it (see how it works in this video using the Chrome plugin as an example - https://www.youtube.com/watch?v=TfslbgUv4F4 ).
Although you can export all your data in Settings -> Backups, you won't be able to import it to Google Tasks, though.