Well, possibly the nuke changed things. He needed more, or something different. So he came back to get what was left.
Not buying bleeding-heart Kellhus, he literally turns his back on Esmenet and walks away.
Ciphrang-Malowebi (to get Zeum) and Meppa seem to be what he was really after.
Ciphrang Malowebi is a pretty huge event. He's only got two heads, so using one on Malowebi must have been extremely important - worth a journey of a thousands of leagues.
Using that to force Zeum's hand post-nuke - that's a theory I like the sound of.
But, Zeum is far away, no matter how you slice it. If there is a path where they can get through the mountains to the North of Domyot, they're still very far away, and I believe its starting to become Fall, isn't it? So snow will be a problem, especially for a desert peoples.
Alternatively, they could sail north and part near the mouth of the Leash. Sailing, I beleive, would be faster than marching, but even if they landed in the Leash they are still on the far side of Agongorea - a similar distance to where TGO is currently.
I don't see how Zeum can reach Golgotterath in any timeframe that allows them to help TGO. Maybe if the siege of Golgotterath becomes a months long process (lets ignore the logistics problems of that for now) then Zeum might be able to show up... But that seems like kind of a waste. Even if successful, TGO at this point will be largely destroyed. Only the shattered remains of a twice shattered army.
So then, whats the plan for Zeum? To come take over the three seas and secure Kellhus' throne? Seems unlikely, what's worth saving?
Is Kellhus just looking for a safe place to lick his wounds post Golgotterath? I guess Malowebi could condition the ground for his arrival in Domyot. That again, though, seems unlikely. A waste of a precious artifact
I just don't see the end game. The path is too narrow!