For the data transfer link between the drone and the command server, are we required to use Wi-Fi? Or is that up to our discretion so long as our solution includes the necessary components (at both ends) for this data transmission? 

We cannot guarantee Internet connectivity at the Stage 3 final event. If you plan to use a cloud-based component, consider designing an offline backup alternative to collect the required data. You may use other wireless protocols or methods to link the drone and the command server. The only required Wi-Fi link is between the sensor modules and the drone.