Most apps interested in the back camera just use the first default camera. On Nx7 these will end up using the only camera available.
If at all they had bothered to do any special check, they'll soon alter the logic. You can always contact the respective devs ... their mail id is published in Play/Market app. Would just be a couple of lines of change.

USB OTG: Supported ... but be aware that just mice and keyboards will play nice for now as other software drivers aren't part of Android unless you/community root and customize.
    http://www.anandtech.com/show/6061/mhl-and-usbotg-on-the-nexus-7-mounting-usb-storage-not-supported

Don't exactly understand USB-NFC adapters!

Anyway though NFC can theoretically work as a communication interface, it is typically used to exchange just a few bytes. You might have better luck with Bluetooth peripherals. There are some devices where Bluetooth pairing can be automated with the help of NFC. Otherwise all you need to do is click a couple of UI buttons to pair.