There is a possibility that this 'egg' is a coconut. The word Metoro used at G18 was niu. And this word has the meaning of both 'coconut' and 'spinning top'. In the Hawaiian New Year there is a ceremony of 'breaking the coconut'.

The henua in P might be the first of the four quarters. The limbs of a human being are also four, with the head ruling at the top. In the niu glyph there are four limbs at top like the four edges going out from the top of a pyramid.

The pyramid is a map of the earth and its top is the north pole. The earth is spinning and then the pyramid / niu also should spin:. That, I believe, is what GD18 also does.

The quarter of the year as a henua-leg might explain also the two 'running sky' glyphs at the beginning of Tahua:

But these two henua might mean half-years, because the four cardinal points are shown ímmediately afterwards.