1) Azure Front Door is not a true 'Load Balancer', but it can distribute traffic based upon 4 methods: Latency, Priority, Weighted, and Session Affinity. If you would like to know more about Azure Networking's Load Balancing options, here is a great doc that outlines the pros and cons of your options.
2) Azure Front Door allows for a quick DNS based failover in the event that a deployment of your application fails. In order to achieve high availability in this method, you will need 2 independent deployments of your application, health probes to detect if one of your deployments fail, and routing configured to direct traffic to the working region.
3) I am unsure what you mean by this. AFD WAF is integrated in the AFD. If the WAF fails, your AFD will also fail. AFD is deployed on a highly redundant infrastructure, and a service failure is highly unlikely. If you would like a redundant or backup WAF, it will need to be configured independently of the AFD.
4) Here is an example architecture that uses Azure Web Apps to deploy a highly available application using Azure Front Door.