Is it possible to use theViewer Offline?
Short answer YES :)
Of course you have to be online the first time when you start the app on your device so it can connect to our servers and download your content ,but as soon as your VR tour is on your device it's being cached by the app and you will be able to use it offline in the app.
Of course Shared Experience will require an internet connection and also Voice Assistant but the rest will work fine even without internet.