It looks like my reconstruction of the Horus myth is missing elements. A fragment assigned to Plutarch says:

When [Horus] had killed his mother to avenge his father, one of the older gods gave as his judgement that they should remove his fat and flesh but leave his blood and his marrow, because the two former had been formed in his mother, while the latter had passed into him from his father at his begetting.

More, in the On the Generation of the Soul in the Timaeus, he says,

This is that therefore which the Egyptians intimate in their fables, feigning that, when Horus was punished and dismembered, he bequeathed his spirit [pneuma, "breath"] and blood to his father, but his flesh and his fat to his mother.

I am told that there is more information concerning this myth in the Papyrus Jumilhac, but as far as I can tell, this papyrus has never been translated into English. I found a French translation, and the machine-translated section of relevance is as follows:

Ra and the Ennead, having learned of this, were extremely angry and indignant. And Ra said to the Ennead, "As for his flesh and skin, his mother [created] them, as for his bones, [they exist] thanks to his father's seed. So let his skin and flesh be removed from him, his bones remaining in his possession." The punishment is carried out. "Then [Ra] went on his way to Dounâouy with the gods of his retinue, Thoth being at their head, [Horus's] skin being with him."

So it seems the courtroom scene of the gods has more to it: while the gods rule in favor of the legitimacy of Horus's rule, he is nonetheless "punished" for his assault upon Isis, resulting in the loss of his Isaic (material) part, leaving him solely with his Osiric (spiritual) part. This is similar to my interpretation, but not identical to it (since I have Horus losing his material part after his second battle), so I have some further contemplation ahead of me.

To be honest, the whole scene now seems pretty reminiscent of the Oresteia...