I would like to try and understand the flow of the traffic. I was using OpenDNS, and selected OpenDNS in the pihole install. Will ALL traffic still traverse through the OpenDNS server? Is there a way to add other DNS servers?
Yes. Your requests will be sent to OpenDNS for resolving.
If you want to bypass the public DNS servers, you can install Unbound alongside Pi-hole.
This will send your requests to the root servers directly and provide you a bit more privacy.
Use this official guide in order to install Unbound on your device:
In this picture at the bottom: https://docs.pi-hole.net/guides/unbound/: is there a way to add more than two IPv4 addresses?
Yes, you can add more than the one there.
You can tick as many as you want and/or use the 4 custom fields available there however keep in mind that anything that’s outside the realm of your network (a public DNS for instance) had the potential of being tracked, monitored and so forth by entities outside of your control.