You click on the username and then submissions, you get all their submissions. Is it what you are looking for?
I don't think HN provides RSS feeds for /comments /posts of user, can't find it at least. So the only option is to scrape the pages yourself for a given set of users. But that's annoying.
I wonder why HN just doesn't allow to follow users and have a new tab added to top bar with /follows. I can't be alone in wanting this. And it doesn't devalue the simplicity of HN in any way nor hinder the experience imo.
You know when you click on the bookmark. You check the bookmarks at your leisure and read anything new. If there was nothing new, nothing to read.