The battery disconnect is the tiny, recessed, round button under the Sim Card/SD Card flap. With no charger connected, use a pen tip or paper clip. This completely (albeit temporarily) disconnects the battery and is the ultimate "shut down".
Dev mode - I should have called it Bootloader mode. From Battery Disconnect shut down, Hold Rotation button and press Power button for approx 2 seconds. You'll see that the red light on the back will light, but you'll likely not see anything on the screen. This is used when managing your device from a terminal on a different machine while connected via USB. Anyways, nothing I recommend to do here. Just forcing the device to go into other modes might help your issue.
I would use the battery disconnect button again to exit bootloader mode, or else long-holding the power button should do it.