You don't need to sign up for each instance (server). You only need to sign up for one. When you login to that one and search from it, the results will include things from other instances (servers) as well.

And if a search result is a community (subreddit), you can subscribe to it as well. They will show up in your feed next time you refresh. If a community says "subscription pending", just ignore it. You are already subscribed.

yes, you can. you only need to sign up for one instance, thats your "home base" and you can see everything from everywhere else from it. it's like you can make a gmail account and still email ymail, yahoo, outlook, whatever else