I'm looking for a feature like local override, but I need it to work without developer tools open. I want to load a local file instead of the file from the server, or just execute some JS on the web file BEFORE render. Can I achieve this with injected JS?
You can use a service worker to intercept a specific network request for a resource and respond with whatever you want instead (a cached copy of your local file or a live response from a local server you're also running). From the linked page (emphasis mine):
A service worker is an event-driven worker registered against an origin and a path. It takes the form of a JavaScript file that can control the web-page/site that it is associated with, intercepting and modifying navigation and resource requests, and caching resources in a very granular fashion to give you complete control over how your app behaves in certain situations (the most obvious one being when the network is not available).