Wife Swap

Question: If Metis comes back, what happens to Hera? Metis is the rightful wife of Zeus, so does Hera respect the sanctity of that union and back off, or fight for the sanctity of her own union?

Well, there are a few issues at work here.

The first is whether or not Metis actually ever was Zeus' wife. Hesiod does in fact refer to her as the first wife of Zeus in one passage. Apollodorus, on the other hand, claims that Metis tried to escape Zeus by shapechanging and that her conception of Athena was via rape, and nowhere mentions any permanent bond between them. Whether or not you consider them to have been married in the first place is really a matter of which source of the myth you prefer. It's not out of the question for Zeus to have married an ancient Titaness, especially one as useful to his newfound need to hold the throne as Metis, but then again the seek-and-seize attacks on women who then bear his children is one of Zeus' most traditional methods of relating to females.

If you do consider them married based on Hesiod, however, he also gives you the answer to the second part of the question: he, like everyone else, also calls Hera Zeus' wife, which he would not have done if she had been in any way considered not his legitimate spouse. Most of ancient Greece was monogamous, so it would not be likely for any of these ancient writers to consider Zeus married to more than one goddess at once. That means that if they consider Hera his legitimate wife, Metis must perforce no longer be considered so. That's most likely because Metis is effectively dead in Greek myth, because Zeus went ahead and ate her and that shit's pretty final; or, since most Greek deities weren't considered capable of dying, it might just be that the act of devouring your wife is a pretty clear declaration of divorce. Greek dudes could divorce their wives pretty easily, especially if there were problems with fidelity or children, and it's pretty clear that the ancient Greeks considered there to be an official break between Zeus and Metis at some point. Because, again... he ate her.

So no, Metis returning would not suddenly mean that Hera's position was necessarily threatened. She is Zeus' current, legitimate, by-law and by-bond wife, and the presence of Zeus' ex-wife (if you so assign her) won't suddenly change that. That union was broken before she ever married Zeus, or else she wouldn't have been able to marry him at all. It was totally possible for Greek men to have concubines while married, provided they didn't live with them and potentially disrupt the family unit, but there's a massive avalanche of evidence in Greek myth that Hera is definitely a wife and not a concubine, starting with the fact that no one ever fails to list her as his wife and ending with the fact that as the goddess of marriage and domesticity it would be massively counter to her very nature to fill that role. Zeus was once married to Metis, perhaps, but is not anymore.

That doesn't mean there couldn't be dramatic problems, of course, especially if Zeus decides he's interested in Metis again or vice versa. Zeus has the same problem he originally had with Metis - sleeping with her might lead to his downfall - so he may be able to control himself, and even if he does go for it he probably wouldn't divorce Hera since he's stuck with her through massive gymnastic numbers of affairs, but that won't make his potential interest in her less likely to piss Hera off (and maybe Metis, too, if Apollodorus is right and Zeus assaulted her). If Metis decides she wants Zeus again, we might be looking at an epic battle of Manipulation between her and Hera, as she tries to sway him to return to her and Hera tries to defend her marriage. Other gods might get involved, too, with those hoping to see Zeus go down trying to help get him together with Metis and those trying to protect him from himself doing their best to keep the two apart.

But as far as Hera goes, her marriage with Zeus is completely legal and current, and it's in her character and nature as a goddess to fight tooth and nail to keep it that way. Metis can't supplant her by just showing up, and is in for a rip-roaring fight if she wants to try to change the status quo.