hey @martin ➬ ,
yes, pubs are necessary to get other people's messages! You could still probably unfollow your first pub by now, because your sbot has learned about many other pubs by now and periodically connects with them as well. Those are all in your gossip.json. Short of knowing a peer's IP address, you can only get messages from them directly if they're in the same local network.
So the short answer to your questions is:

1. probably, because your sbot knows about other pubs
2. it remembers them in gossip.json
3. no, not this specific pub necessarily
4. yes