Once a dark conscious thought is within the thought plane, will it also then influence the deep subconscious or even subsequent conscious thought?

Thoughts being energy, they will have a local effect most strongly, and this is the reason you perceive this as hovering about the head. That is an intuitive awareness that the thought plane is not some far-away repository but an energy field that surrounds the individual’s faculties and interacts with them. The primary purpose is to be a local receptacle and that is most efficient when the energy fields are in proximity. The liability is the consequences of too large a build-up of negative thinking. This has a silver lining, so to speak, in that most people, when sensing things are getting too dark, will seek relief in some way. The tragedy is when they cannot find it. But the mechanism to know that something is amiss does serve. So this is not a fault in the design. The fault, if any, is with the thoughts themselves, whether they are an authentic response to reality or an exaggeration, whether they are occurring because the person fails to take action to save themselves, or may be in a hopeless situation beyond their control, but nonetheless, the stakes are high in life always with respect to what can happen for good or for ill.