Service Worker Response Cache Headers

A) is the correct model. If a service worker controls a page, all network requests will trigger the fetch event handler of the service worker prior to consulting the browser cache or the network.

In turn, any time the service worker makes a network request, either explicitly via fetch() or implicitly via cache.add()/cache.addAll(), the browser's "traditional" cache is consulted for a response first. A network request to the server will only be made if there isn't a valid response in the browser cache.

This sometimes works in your favor, and sometimes can expose you to subtle bugs if you don't expect that behavior.

There's a very detailed explanation of this interaction at https://jakearchibald.com/2016/caching-best-practices/, specifically covering things to avoid and ways to take advantage of this behavior.

That depends on how you configure request. With Fetch API you can control how request interacts with browser HTTP Cache.

For example you can set request's cache mode to no-store so it will bypass HTTP Cache. Or you can set request's cache mode to force-cache so browser will return cached response even if it is stale:

fetch("some.json", {cache: "force-cache"})
  .then(function(response) { /* consume the response */ });

By default request's cache mode is default. In this case request will act as usual. Obviously that is only if service worker actually performs request instead of returning some hardcoded response.

For more information check Fetch Standard, Request.cache MSN page and Using Service Workers MDN page.