Yes. Exploria is a free service for audiences and readers.
Yes. Exploria has been built to work on the most popular versions of Apple's iPhones and iPads as well as phones and tablets which run with Android.
Yes, but only for Stories, not Challenges which require an internet connection.
When you search for stories, the playlist or list of stories, and their text is downloaded to your phone so you can view or listen when you don't have reception. When you're leaving a town, make sure you have your playlist ready so you can listen to Stories if you are offline.