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

When we say ‘pub’, what we really mean is ‘a peer available to peers outside of the local network’, don't we? NAT is a concept from the IPv4 world, and it only concern incoming connections. IPv6 peers – whether they are pubs or clients – may be available to anyone with an IPv6 connection. The same goes for cjdns, yggdrasil, etc.

My IPv4 address is NAT'ed (but I have opened for the relevant port in the router and I have a static IP address, so…), but even if it weren't, my sbot would still be able to reach out to any other IPv4 peer (that accepts incoming connections, whether a pub or not) that it has heard about on the grapevine.

I haven't been following any pubs for months, and I don't feel like I'm missing out on messages (and several tallyings of unique IDs, that I'm too lazy to search for now, seem to confirm this).