Your questions are quite complex and, in fact, combine several ones. To address them clearly, I will begin by explaining briefly what Fu Xi and King Wen represent.
Fu Xi represents a heavenly principle. His system reflects a creative, primordial arrangement — a pre-heaven concept that expresses universal order before manifestation.
In the arrangement of Fu Xi trigrams, we see an ideal situation — the beginning of existence. It shows how the world existed first as ideas and concepts. Within this arrangement, opposite energies such as Heaven and Earth, or Water and Fire, create vertical and horizontal axes. These axes form the basis of the Universal Square which relates to spirit and form (the vertical axis) and to life and substance (the horizontal axis).
Next, we have the Individual Square, which is made by Dui/ Lake , Xun/Wind, Ken/Mountain and Chen/Thunder. This square reflects how we present ourselves to the world — how we nourish ourselves, how we persevere, and how we function in life. It expresses our individual way of showing ourselves within existence.
King Wen, which is actually older than Fu Xi, represents the created principle. It is more related to time, manifestation, and how life evolves within manifestation. In this sense, King Wen can be associated with branches, whereas Fu Xi is more related to stems. The two arrangements therefore differ.
The King Wen arrangement divides life into three-year periods. For example, the Chen period / Colon, corresponds to the first three years of life. The next three years relate more to the Lung, followed by the Spleen, and so on. This three-year cycle repeats itself throughout life. As an example, Spleen years include ages 7–9, but also 31–33 and 55–57.
Spleen energy is related to self-consciousness and the ability to see purpose in life. If something significant happens during a Spleen period — for example, a parents’ divorce — the energy of the Spleen may be weakened, which can later manifest as Spleen-related problems.
Trigrams are not only symbols for individual energetic aspects of the body; rather, each trigram carries a different quality of energy depending on the arrangement of yin and yang lines. This helps us understand the functioning of stems, branches, extraordinary meridians, and so on, at a very fundamental level.
Therefore, when treating a person, it is essential to understand where the problem originates. Is it primarily on the stem level, or on the branch level? Fu Xi represents the blueprint, so we need to consider whether the issue exists on this level or elsewhere.
Only then can we choose which system to use. The treatment must fit both the chart and the energetic constitution of the patient. There are no fixed rules.
For example, it is important to be aware that whenever you work with yuan/luo points on Kd–Bl, Liv–Gb, Spl–St, and similar pairings, you are already treating within the Fu Shi.