Indeed, I think that was always part of the plan, the going back.
I have a feeling that part of it was to draw out whatever Yatwer had planned. If he had been there more constantly, he wouldn't have known what Esmenet would have done and, indeed, did do. She would have been constantly under Kellhus' thumb. Kelmomas would also have been more circumspect and wouldn't have revealed himself (or Ajolki).
There has always been something of a question in my mind about why he even let Fanayal live for any amount of time. I think the answer came in those last two tent scenes. He was drawing him out along with Malowebi and of course Meppa. Consider if he had simply murdered Fanayal however many years ago. It would simply be someone else there attacking, so it makes no difference. Why allow them to attack? Because all those Fanim warriors need to see that even with a god's help, they can't win.
I do think the Ordeal is a throw away, but not entirely. The Ordeal matters, but only the Few among them. The rest are only there to deliver them. We are literally told this in WLW. And as MSJ points out, the Ordeal's journey becomes Holy. In other words, they are on a holy quest. The end is something holy. My guess is that Kellhus installs himself as the True Solitary God, or some such. Probably through the same means as the No-God is made. This is why he needs to get to Golottereth. This may be why he didn't outright kill Meppa? Water will later become, literally Holy?
The qeuestion I don't really have an answer for is why does he need Proyas to "do what no believer could do?" All I can think of is that he will renounce Kellhus, but I don't understand why that helps him. Or is it that he wants him to do everything believing that Kellhus has really forsaken him. In other words, not rely on him?