ESD protection of battery operated device in metal case
Whatever your doing, if the device is in handheld mode or plugged into the USB, and ESD currents should be redirected from the PCB on the inside. A metal case is better than a plastic one, and the problem is most likely due to the inputs. 

A PCB surrounded by a metal case is in a very good condition, as the case acts like a faraday cage. An ESD event or spurious voltages will met the conductor of the case and be redirected. The problem is most likely due to the inputs. Any inputs to the PCB should be protected with diodes, to shunt any high voltages to the chassis of the device. The device chassis could be considered a ground while floating or in 'handheld mode', we do this with cars all the time. 

You should have some kind of ESD protection on the USB end also and on the video end. The shield of the USB should be connected to the chassis. The 'video end' most likely has shielding, if it does these should also be connected to the chassis. That way if ESD hits the chassis with USB or video connected, the current pathway would be through the chassis and out through the shield. 

You may also want to preform an experiment. Get aluminum foil or copper tape and wrap the device ports (video and USB) (but not the chassis). Then handle the device in a similar manner when the failures were experienced. If the device fails, then it is probably not due to ESD and may be something else. My thoughts are that the device would be fine in this situation and will operate properly. 

It may also be advantageous to experiment with the chassis tied to the PCB ground at a reasonable point.