You are correct the FIREWALL_VPN_INPUT_PORT is to allow the port to be opened on the vpn side of the connection. You will add the port provided to you from Air as that variable and set the same port within qbittorrent.
You should not be adding the port 6881 anywhere, example compose below from the guide on my website.
https://paste.drfrankenstein.co.uk/?6be32ded3f56a96c#9q46W92Ah424RMoB4qM2o8UHL1CBxickoZKZgH9Nsfop
Hey, unlikely as the wan traffic should be going via the vpn. But at least you set it correctly now.