As far as the subscriber issue "Should a guest author first be a subscriber to be considered?" I get many unsolicited article submissions from people who are not regular readers of my blog. It is very annoying. I wish more would subscribe to my newsletter, rather than dropping by with a guest post and leaving (especially if it's not useful to my audience). But that's not usually how it works.
More often than not, I get emails with article submissions and proposals from people who barely read my blog.