If the overrides don't contain the HTML for the feed link, you can just grab the HTML from the source templates in K2. Or (most probably) they are hidden either in K2 (check settings and categories and menus) or just using some CSS.

I kinda lost you on sorting the RSS feeds by category...

The easiest way for you to create a custom RSS feed from any categories (and unlike Joomla's com_content which is still rocket science in 2018) is this:

- Create a menu in Joomla that you don't retrieve anywhere in the frontend (let's call it "Hidden").
- In that menu, create a new menu item pointing to K2 Categories (jmp.sh/ZNor0F3). In there select the categories you want. For new K2 menu items, the RSS options are enabled by default.
- Before saving, make sure you add a descriptive yet short menu item alias, e.g. "allnews". Save the menu item.
- If this is a top level menu item, simply go to the site's frontend and call: /allnews?format=feed
- Done.

K2 uses the convention in RSS URLs that nearly all components that support RSS output use and that's the "?format=feed" when you are in itemlists, be it a category, tag or user page.

However, unlike other components, K2 can also output JSON which means it's dead simple to create a mobile or JS web app purely out of JSON data. Just append "?format=json" at the end of *ANY* K2 URL, even an item.