When I take power away the devices stop being connected to the N7. I will make a quick video demonstrating to maintain the plausibility of my sanity.
When you switch power back on in the video, your N7 screen goes off just a few seconds later. When using the fixed installation kernel and the same power source for N7 + hub, the return of external power will normally reset the N7 screen-off timer and your screen should stay on, at least for 15 seconds longer (depending on your configuration). I really don't understand why you want to individually power your N7 and fiddle with "shared ground" issues and such. Depending on your hubs ability to use a dedicated power source, there are two options available:
Option 1: Use this when your USB hub does not provide an external power connector. Connect external power directly to arm 1 of your OTG Y-cable and connect your USB hub to the other arm. Power will flow to hub and N7 in *parallel*. You can disconnect your N7 and power will continue to flow to your hub and to all connected slave devices.
Option 2: Your USB hub provides it's own power connector. You can still use Option 1, but you can also feed power directly to the hub. Then use an OTG Y-cable to make your N7 become the USB host (arm 1) towards the hub *and* have it draw power (arm 2) from one of the hub's slave connectors.
In regard to the "ignition-off postpone" functionality: this should work with both options. But with option 2 your N7 would also benefit from the extended power time. If for whatever reason you need to use a different setup and can't get it to work, then please provide a circuit drawing.