Since the pi itself is using a 192.168 ip can I use the 192.168 killswitch?
I think you could select the 192.168.0.0/16 as PiFi runs 192.168.3.14 IP, which falls within the 192.168.0.0/16 range
However, PiFi already has a WireGuard kill switch will handle any disconnection, ensuring that no traffic leaks through if the VPN drops. In this case, you can disable the Mullvad kill switch, as PiFi’s kill switch should provide the necessary protection.
That being said, it sounds like it would be fine to enable if it gives added peace of mind