To allow connections to the web server, all you need to do is open the OCI firewall (security list) for http(s) ?
You must also open connections in the local firewall of the compute instance.
A web server is required to interpret PHP scripts included in WordPress. In this Learning Path, you used Apache, but other web servers, such as Nginx, can be used.
Yes, WordPress requires a RDBMS. The preferred database to use with WordPress is MySQL.