Is the reason that the upper subconscious usually cannot recall past life trauma memories because it cannot actually review the akashic records itself? Some people can recall past life memories via their upper subconscious. When that happens is that because they are past life memories already corded to cellular memory? If so, how does that cording come about, is it created by the deep subconscious, or by the Law of Karma?

Both things can happen. The deep subconscious can direct its awareness of events on other timelines to the upper subconscious and create a cording for a memory it has tuned into within the akashic records. So this is an active dynamic, explaining why people can be haunted by things and then, on reflection, may see disturbing images or have unusual ideas come to them, which are difficult to understand when it is past life material, especially. The Law of Karma will create cordings in response to ongoing events in the current lifetime that are a kind of resonance with prior difficulties and karmic obligations. And the Law of Karma will ensure a cording to that level of the mind experiencing the current struggle, whether it is the upper subconscious, cellular consciousness, or the deep subconscious, and it could cord to all three, particularly when multiple levels of the being are juggling the same set of circumstances and issues. This is more likely to happen with serious matters, in keeping with their larger energy and the pressing nature they represent to the Law of Karma in wanting to get on with it, to get some kind of rebalancing going. And this will trigger a person to feel something in the way of symptoms, and that will start the ball rolling. Then, the consequences will begin to emerge depending on whether a person can recognize this as an unmet healing need and pursue finding relief, or tries to ignore it, or keep it bottled up, in which case things may worsen.