Implementing Service Workers with an appcache fallback for unsupported browsers

if a browser supports service workers then the service worker caching will be used instead of the appCache manifest. You can include the appCache manifest for legacy browsers like Safari and things will work the way they did in the past. Plus for modern browsers they will leverage service worker caching and act as if the appCache does not exist. Sort of like the way responsive images work.

The check which technology the browser is supporting is easly done:

if(navigator.serviceWorker){
    initServiceWorker()
}else if(window.applicationCache){
    initApplicationCache();
}else{
    console.log('no caching possible');
}

Dynamic loading a service worker should not be a problem since it is done in javascript anyway.

Dynamic loading applicationCache's mainfest seems not to be possible, but you can try an iframe hack, see: Dynamically Trigger HTML5 Cache Manifest file?