There is an adapter in the box with the 2016 VR.
No there isn't - at least there was not in mine which I got when the 2016 was first shipped with the S7.
And I think that the S8 USB type B to USB type C adapter will make the S8 too big to fit into the Gear VR 2016 (a dangerously tight squeeze at best for the S8+, I'll try my S8 later today and update - UPDATE - No: Not a chance of it fitting)
The New Gear VR 2017 has a changeable adapter which can be switched from USB Type C to USB Type B, so it is backwardly compatible. Apparently this is the same as the Gear VR that was shipped for the Ill fate Note 7, so I am going to refer to this as the Gear VR 2017 for the sake of brevity.