Device enumerating at USB 3.1 (The specification has been updated to USB3.1, defining Gen1 (5Gbps) and Gen2 (10Gbps)) first enumerating in USB 2.0 mode.The Host switches to Superspeed mode only after retrieving device configuration descriptors in USB 2.0 mode. So, USB 2.0 must be supported, i.e. wires connected.
It is not allowe by spec to not support the USB2.0 functionallity:
220.127.116.11 Peripheral Devices
A USB 3.1 peripheral device must provide support for both Enhanced SuperSpeed and at least one
of the USB 2.0 speeds. The minimal functional requirement for the USB 2.0 speed implementation
is for a device to be detected on a USB 2.0 host and allow system software to direct the user to
attach the device to an Enhanced SuperSpeed port. A device implementation may provide
appropriate full functionality when operating in the implemented USB 2.0 speed mode.
Simultaneous operation of Enhanced SuperSpeed and USB 2.0 speed modes is not allowed for
USB 3.1 wires can be left unconnected - this is identical to attaching device to Host USB 2.0 port.
For the lightning question, it is correct that the Ipad pro supports 3.1 Gen1, according to the internet. Now the current lightning cables only have the limited amount of of data lines and therefor they do not support USB3.1 Gen1 but only USB2.0 High speed.
I am currently not sure if Apple is planning on bringing modified Lightning Cables to support USB3.1 Gen1 and therefor the converters to USB Type-C connectors/cables