The Reverse Proxy mode is the most appropriate FortiWeb operation mode for this scenario. In this mode, FortiWeb sits between internet users and the Linux web servers, terminating client connections and then forwarding requests to the backend servers. This enables deep inspection, protection from web attacks (like SQL injection and XSS), and full WAF functionality, making it ideal for securing front-end web servers exposed to the internet.

Azure ExpressRoute provides dedicated private connections between on-premises infrastructure and Azure data centers, bypassing the public internet. This results in more predictable latency, higher reliability, and better security, making it ideal for mission-critical workloads.

The branch-to-branch topology in Azure Virtual WAN is characterized by direct connectivity between branches through the Virtual WAN backbone, which reduces dependency on centralized hubs. This results in a simplified network architecture, lowering latency and optimizing routing between branch locations.