1: The point of wifi calling is to assist when there is low or weak cell service. Thats was the whole premise of it becoming a thing. Also Verizon states this as the reason:
"Once you set up Wi-Fi Calling, you’ll make Wi-Fi calls like you’d make traditional voice calls.
In order to provide you with the most reliable experience, calls will use the cellular network when available. If you’re using an Android device, then you can change the preferred network when traveling internationally. For Android devices, you’ll see a Wi-Fi Calling icon similar to this:
when your call is over a Wi-Fi network. If you have iPhone, you’ll see "VZW Wi-Fi" in your status bar."
2: I stated ATT and Verizon. Apparently this changed with T-Mobile in this beta as I am understanding it.
3: The release notes make no mention of wifi calling. Not sure where you got that info.