Ideas:

1. Now we arrive at the end of the week. Might the elbow-adornment be a hint of this? It is closed. There is no way out.

2. The shape of the adornment - like an inverted V - is similar in all the 5 true planets, but different in Sunday and Monday. In the rongorongo texts similarity in shape means similarity in other aspects too (I believe).

3. All the 5 true planets do also share the similarity in not having any feet ("for they were round like an eye-ball and climbed the mountain by rolling over and over, as they had no legs"). Instead their legs look like an inverted V.

The rule of similarity tells me that this inversion might mean that the true planets get nourishment from below, given that a not inverted V (as seen in the left hands of Monday - Wednesday) means receiving lifegiving light from the sun. Thirsty creatures can find life-giving water in the underworld. Even the sun refills there. Maybe the feet of the planets are invisible because they are recharging while standing in the water?