Consider I have:-
myapp.firebaseapp.com with it\'s own manifest.json.myapp.firebaseapp.co
Sure; the manifest for a given page is determined by the presence of a DOM element with details about the manifest's location.
Assuming you have full control over the HTML for /admin and for all other pages, you can include in the HTML for /admin, and in the HTML for other pages.
Each of the manifests can include, e.g., a different start_url value so that they take the user to a different initial page.