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There's actually a supplemental spec, USB-PD(Power Delivery) which now accopmanies USB 3.0 and addresses this. It is carried over the USB physical layer but does not rely on the data layer for communication.

According to what I can find[1][2], USB 3 supports 100W and a standard C connector supports 5A.

[1] http://www.usb.org/press/USB_Type-C_Specification_Announceme...

[2] http://www.usb.org/press/presskit/USB-IF_IDF_Shenzhen_2014_P...

http://www.usb.org/developers/powerdelivery/

It does, but it's not a requirement for all devices so you may or may not need a hub to provide power back to the laptop depending on the device.

https://en.wikipedia.org/wiki/USB#USB_Power_Delivery

To answer more specifically; yes— not only is every node in the bus network addressable for power needs and capabilities, even each of the two ends of a cable is a separately addressable node (mostly used to figure out which way a swappable cable is oriented, I think).