If I understand the FI kernel correctly, it is designed to NOT provide any power (out) from the USB port upon AC/USB power removal (so the N7 battery is saved) AND to place the N7 into a deep sleep state (after a certain time).
-Is this correct?
Yes, it is. But a fusspot like me would prefer to phrase it this way: The FI kernel is designed to NOT provide any power (out) from the USB port upon AC/USB power removal, (not to immediately prevent battery draw, but) to allow the power management system to place the N7 into deep sleep state, after a certain amount of time, in order to then prevent lots of battery draw over longer periods of time. While supplying battery power to slave devices, a N7 will never transition to deep sleep.
-How long after power is removed should the N7 take to go into the sleep state?
This is a good question. First: the individually configured screen timeout. The screen needs to go off, before we can even talk about deep sleep (but you can also bypass the timeout and shut the the screen off manually). The device is now in light sleep mode. What happens next, is a little difficult to say, because some not so apparent rules will come into play. The transition to deep sleep can start as quick as 10 seconds later, but it can also take up to 300 seconds to start. It's a bit like with quantum mechanics. As soon as you try to really witness the process, you are likely to manipulate it. Wake locks can prevent all attempts to transition a device to deep sleep. Creating an adb-over-wifi session will raise a wake lock, which will prevent the device from going to deep sleep. In fact, every installed app can do this. Unfortunately, from the outside, it is not possible to say, whether your tablet is in light sleep or in deep sleep mode. One way to judge, is to check the amount of battery drain after some time. As mentioned earlier, the amount of battery drain in deep sleep mode should be very, very low. If you loose more than 4% in 12 hours, your device is not going into deep sleep and you may want check your list of installed apps and services.
If you really want to find out more, you can search your kernel logs (fetched afterwards) for the following entries: "suspend: enter suspend" and "Exited suspend state LP0". The latter is usually followed by two interesting entries: "Resume caused by ..." (for instance "tegra-otg", which would indicate, that wake was caused by someone unplugging the OTG adapter) and also "Suspended for nn.nnn seconds". The former MAY be followed by "active wake lock PowerManagerService" and "Freezing of user space aborted". Which would indicated, that an attempt to transition to deep sleep has failed. Which may or may not be reasonable, depending on what apps are trying to do.
With this information you may be able to find out what apps are preventing deep sleep mode. And any such finding could be beneficial to other users.