Marina,
For instance, I still haven't been able to figure out how to follow or
mute topics and hashtags from the web.
I agree, Mark really needs to revisit this UI. The controls and the effects are almost completely hidden to someone working via the web site - they exist only in the Groups.io home page list of all your groups.
At the bottom of each message in Digest there is a Mute this Topic
link but not a Follow this Topic link.
Mute and Follow are not opposites (they're unrelated).
The links to Mute a topic or hashtag are shown when your Message Selection setting (under Advanced Preferences in your Subscription page) is "All Messages". That's the default, so Mute Topic and Mute Hashtag are the only links most people see in email. The opposite of Mute is Unmute, and you won't (or rarely) will see that one because you won't be sent the messages it would apply to (except if they're special notice, I think). So unmuting a topic or hashtag is an action where one is forced to use the confounded web interface.
The links to Follow or Unfollow a topic or hashtag are shown in email only when your Message Selection is "Following Only". That's a very powerful feature for limiting how many messages you receive in busy groups, but I suspect few have figured that out.
"Chosen to follow" means "that you have chosen not to mute"?
Nope. Chosen to follow means that you Followed that topic or hashtag.
Working by email you might wonder how you ever follow anything if you're Following Only. That chicken-and-egg problem is cured by the "First Message Also" checkbox. You receive the first message in each new topic, and in that email you have the Follow links. If you're not interested in following that topic do nothing and you won't receive the replies to it. If you do follow it then the subsequent messages will have an Unfollow link, for when you tire of the topic.
Shal
Shal Farley