Should the focus be placed in the right ear where there is the constant buzzing sensation or at the back of the head where the vibration is more like a constant dog whistle?

If there are two or more naad sounds, the yogi should determine which is the deepest. That is the one to select for focus with observing where the other sounds are located and what frequency they emit.

 I also find that repeating Om Namah Shivaya mentally on the exhale and Om Tat Sat on the inhale during my meditation helps to deepen my focus on naad--sometimes I slip into a state where I lose connection to the physical body (hopefully that makes sense). I am not sure if mixing the mantras is a good idea, but the need to do it popped in my psyche. I generally do not focus on the coreSelf while focusing on naad, but I will make the attempt.

If you were inspired to use a mantra or a combination of mantras, try to determine where the inspiration came from. This is important because in the future there may be other inspirations and identifying these will give you the ability to determine which are reliable and which are not. There are many voices there out in psychosphere. You should know which contacts you and which suggests methods to you. If a certain inspiring force influences you and you find that its advice works for you, then in the future when you are touched by that, you may eagerly accept it. In contrast if another inspirating energy influences and you find that its advice leads nowhere, you may recognize that one as one which you should ignore.

As for the light, I have experienced a flicker of light in the head--it's not a penetrating single light--it is as if a smokish whitish light flashed (not very bright) in the middle of the head and then disappeared--similar to a lightbulb that flickers in a dark room. I am not sure if this is the same light,

It is different to this light(s) you experienced