u/DragonlordHML

It looks like Sunny may end up getting thrown into the distant past of the ToA while killing VTB.

This makes the possibility of Sunny natural reaching apotheosis very high. And it looks like Nephis is not reaching natural Apotheosis.

As for not falling into Corruption while reaching Apotheosis, three possibilities come to mind:

1. Millennia of Weaving allows Sunny to make a memory that holds off his Corruption while he attempts Apotheosis. (Unlikely)
2. He gets thrown so far back that Corruption has yet to encroach upon the Tomb of Ariel, thus allowing him to attempt Apotheosis without fear of Corruption.
3. Back when Sunny, Neph and Ananke were entering the Estuary, Sunny mentioned that although he was a shadow and should have resistance to Corruption, he was still partially human.

Natural Apotheosis involves losing your humanity, thus once he fully becomes a shadow, he would probably be nigh-immune to Corruption.

Edit: remember that the ToA is essentially a Realm of its own, isolated from the Dream Realm.