If I use wifi calling and leave wifi will my phone switch to cellular calling?

Yes. It will fall back to normal GSM or to VoLTE if that is enabled on your phone. You don't have to turn off Wi-Fi or Wi-Fi calling.

Yes, it will, as long as you have cellular service. Wi-fi calling will only work when you are connected to a wi-fi network, and the purpose of that is for when you are in fringe cellular connection areas. The phone will still work with your cellular network. It is not one or the other.

You could attempt (test) this yourself by disabling wifi on your iPhone during a WiFi Call.