That’s OK to get the SSL certificate for www. as well. Of course, let’s hope the devs will add an easier way in future versions.
No need to rollback. It’s good to have the two certificates, for https://example.com and https://www.example.com, even if you then configure redirections.
Good.
No, you just need to adjust your Ghost (/var/www/ghost/config.production.json) and Nginx configurations, and restart. See an example with redirections to https://example.com