How do I view/mount nandroid file on device? From Android's terminal, I tried to mount a nandroid img:

losetup -f


gives "/dev/loop0".

However, when I try losetup /dev/loop0 /sdcard/nandroid/... data.img it says no such directory. Same thing when I try to mount the file. 

According to this it should be doable, but it seems there must be some trick to mounting it in Android? https://en.wikipedia.org/wiki/Loop_device
I don't think you should need to use losetup, however, if you are using Android 4.2, the path to your sdcard may be the issue. In Android 4.2, that has been changed due to support for multiusers on tablets. You would likely need the full path rather than the "shortcut" that has been set up for simplicity by Android. That path would be /storage/emulated/0/nandroid/...data.img.

But, I believe Android will support mounting it directly with the mount command (it is what I have used a few times in Linux, although, I've never tried in Android).

mount -o loop /storage/emulated/0/nandroid/...data.img /mnt/wherever

Keep in mind, you will need to be root and the directory you want to mount it to needs to exist before you run the command. And when you do mount it, if you mount it over a directory that has content in it, it will prevent access to that content (it isn't gone, but unaccessible until it is unmounted).