"In over his head" is a saying that means someone is in a complex or confusing situation that they can't understand or get out of. So in this case Baldwins Trump's presidential duties were too complicated for him to understand or to do properly . Another way of saying in over his head is "Out of his depth". Cameras blazing means that the cameras are focused on someone. Blazing in this sentence is being used to stress the use of cameras at the event. In this case they were focused on Mike Pence. (The flash of the camera lights is blazing).