I've never seen anyone say it was cheating. more like haveling a pocket reference manual.
you can get the same info with quick alt-tabbing to 17lands really, it's just convenient to have it as an overlay.
I liked them more for the collection tracking convenience myself. that became a lot harder once WoTC took the collection data out of the logs.
I saw the code for one tracker that basically injects some c++ or unity code or something into the memory where arena is running and gets the collection data that way. it does not seem to work in proton the same way though, at least not for me.
Yes. You can follow a user and get notifications when they post something.
You can follow a user, a community, an individual post, a comment, a topic (collection of communities), and a feed (user made collections of communities).
Then you get notifications for posts/replies as relevant.