The web platform has lots of tech now for fully offline web apps - the currently used term for this is a 'PWA' or Progressive Web Application. On Android and iO

The web platform has lots of tech now for fully offline web apps - the currently used term for this is a 'PWA' or Progressive Web Application. On Android and iOS in many cases there's a little option in your browser to install a website as a PWA, which puts it on your home screen and locally caches the assets. IIRC Twitter and Google Maps both support this for their website. Of course, writing a PWA is still a huge pain in the ass, and it doesn't give you those guarantees you mention from native apps - auditability, etc. But maybe it could get there, who knows...