Hi Luka,
that should be the case, but I have noticed several times that if I touch the camera enclosure with the USB connector, then I can even see sparks
meaning that the USB ground and the power supply ground are not at the same potential... I am powering the camera from a 12 V battery, and the USB is attached to a laptop running on its own battery. Did you notice anything similar?
And don't worry about the PWM TEC control: anyway I think the best option would be to redesign the board... but that is a whole other story!
Nico