Couldn't you use just an extra query string parameter when you want to force a refresh? Like, ?v=12312312353. It's a little hackish I'll grant you, but so is caching HTTP in LocalStorage.
This won't actually cause anything to be evicted from the cache. It just adds a new cache entry with a new key. This is poor resource management, and removes some of the appeal of going out of your way to reuse the browser's cache mechanism.
Sure, but the HTTP cache is managed by the browser. Unless you are pulling down mammoth amounts of data, cache management is unlikely to be a concern for you.
Browsers cache many things each time you go to a website - all the html and assets (js/css/images) will be potentially cached (depending on headers). Often you will not go to that website again. They are optimised for this.
About 'isCacheValid' setting the cache key, you're right, it should have been done in the success callback. The gist is going to be updated soon with your feedback.