No it's not possible. It will automatically pick the source the provides the most power.
Who knows? Apple may announce for macOS 10.16 an ARM version of the OS that can run on existing T2 devices.
You’re confusing Secure Element and Secure Enclave. The Secure Element is used to store Apple Pay information whereas the Secure Enclave is used to store cryptographic keys and is part of the A, T or S series chip (e.g. A12, T2, S2). What’s on the picture is the Secure Element. You can read about both on Apple’s iOS Security Guide (https://www.apple.com/business/docs/iOS_...)
No. Continuity and the Apple Watch Mac Unlocking feature use Bluetooth.
Nah. The MacBook Pro is becoming like iOS devices: outside of backups, nothing will be recoverable.
Given the T2, having a service port or a Thunderbolt port form which data can be extracted would amount to having a backdoor.
It's only for the secure element. If my memory is right, the iPad also has a similar controller but doesn't support NFC.