I got it my Slate and tethered phone working without the powered hub:
- Powered Slate using usb-c charger and usb-c cable and usb-c to micro-b PD adapter
- Connected phone using micro-b cable with micro-b to usb-c adapter (probably not necessary)
Has been stable and keeping phone charged for several days.
Before this I tried powering the Slate using a different 3A 5V micro-b charger and phone would drain over time. Could be coincidence and the micro-b charger just cant deliver enough of the rated power or maybe Slate negotiates a lower power over micro-b somehow and the usb-c to micro-b PD trigger overrides that? Not clear but happy its workign without multiple power supplies now 
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