Without having read Philebus, I will try to answer you the question, expressing an argument hopefully giving you a little more insight on what is going on here.
You try to understand Zeus and Hera by reading Plato, when you should start by reading Hesiod's Cosmogony.
Your thoughts imply that Zeus has no soul himself.
"But mind and wisdom cannot ...
It's been 3 years but better late than never.
Zeus wisdom was always present either within his judgement or by Athena's voice.
We should remember that Metis is the expression of something people had in mind at that time. As always myths are made by people and myths many times represent ideas people of that time had. Metis personifies a set of skills.