When doing traditional HMR, is the “body metaphor” people tune into, to sense where they are holding their issue, actually contained within and expressed by cellular memory alone, and simply translated and related to conscious awareness of the client by the upper subconscious mind?

You are close to the truth here in understanding the peculiar ability almost universally observed with clients undergoing an HMR session, even for the very first time with little prompting about what to expect, that all will have an ability to sense a body metaphor when there is a serious issue they are working on. This is inexplicable to the psychologists who will be quite puzzled about this strange phenomenon, but it is clearly robust and universal. What is taking place is that an emotional issue, and the key here being the word "emotion," does register within the cellular consciousness of the body because it is where emotions are felt. They are not felt within the mind. They are felt within the body and that serves as a painful stimulus accompanying an experience to make it visceral, to make it painful in some way, so it will register as being of high importance to the mind and thus the memory of it happening will have that extra emphasis from what the emotion causes to happen, and that is to register the memory within cellular consciousness, keyed to a link to the emotional apparatus. So there is a potential of recreating the very same emotional response, as in the original experience, if a person focuses on this for any length of time. That is the so-called danger of relive some will exhibit when recalling a memory of an unpleasant episode. Most people coached to avoid lingering on it too long will have enough of a connection to know and understand the list of emotional consequences that took place when the trauma was underway without having too great a physical re-experiencing in the body of the painful feelings. That is an advantage of the HMR process, that it is gentle compared to a hypnotic regression that will put the person in a deeper trance state where the cellular consciousness is reawakened and revisited more directly to view the experience in the mind as though it is happening, and with that will come a relive of the emotional consequences in parallel, and that revivification can be quite intense and even intolerable in many subjects. So what you are probing here is spot on with regard to the true origin of this inexplicable ability people have to connect the memory of a past trauma to the body and, as you have seen in working on your many clients, this can be almost anywhere in the body and felt to be internal to the body, external, or both, because that is an aspect of how it was experienced and the memory retained within cellular consciousness.