I'm pretty sure that regardless of the exact time that Redcloak destroys Xykon, it will still count as turning on Xykon.
Unless you want to pull a Char and claim that Redcloak was never loyal to Xykon in the first place so he can never betray him.
Either way the risk is still pretty great. Whatever the goblin pope's planning to do with the real phylactery, it must end with Xykon destroyed for good.
Redcloak isn't the pope of Mr. Rainbows the benevolent god.
Redcloak is the pope of the Dark One, a god of war and violence that ascended after running a bloody crusade accross the land.
People sometimes seem to forget that Redcloak is evil. What's a few/lots of sacrifices along the road? What's close family compared to your god's direct decree? The goblin pope's even stated to be ready to sacrifice all of goblinkind in his divine mission by just getting the world destroyed and hope the next one is better.
Contrast to Xykon who actually declared he enjoys this world and has no plans to destroy it.
That's still control. In particular considering that there's a plenty long line of corpses of people who tried to manipulate Xykon.
And funny thing is, Redcloak's the even bigger monster here, with a plan that risks ending with both the world and gods destroyed. That's overkill even by Xykon's standards.
So no, even if the OoTS has some big battle against the lich and wins, Redcloak won't go "oh I guess I surrender". Redcloak will go "you just destroyed my loose puppet, now you face me". The real challenge will be to bring down Redcloak whitout killing him.