I don't have experience with the pentax 67II but I've had other cameras fail in cold weather. I do a yearly night photography project in the middle of winter (middle of Canada) and use this as a way to test and break in equipment. I think you've probably just found the limit of that particular camera system, you may try a different battery (although the lithiums you have are usually the best) or keep the camera inside your jacket.
The mostly mechanical Pentax 67 MLU model with waist level finder may be an option for cold weather that would let you use your lenses, although I've never used one in -7C weather.