1. I use RSS for blogs, software version tracking (e.g. the linux kernel), and serial content (webnovels, comics, etc.) I used to use it for news, but the dispropotionate output of different sites cluttered the feeds and didn't work well for me, so I visit news sites manually for now.
2. I use sfeed with dmenu (and read content in the browser). Low resource usage, simple setup, scriptable. I have not been happy with any of the (android) smartphone-based RSS software that I've tried. The interfaces are clumsy, and trying to sort through the giant list of content on a touchscreen with my fat fingers without error was frustrating.
3. Not at all. I don't use any of the big social media, and for more niche social media (like HN), I don't use RSS.
4. This is not a problem, in my experience. RSS/atom are pretty simple and have implementations in nearly every widely used software stack on the web, so pretty much every site has an rss feed (although they don't always advertise it!). However, if I absolutely needed updates on a specific site that didn't have a feed and didn't care enough to manually check it everyday, I would probably just use curl to scrape it.
5. Yes.