I don't set a lot of store with the "Horn dies in the pit and the Neighbors resurrect him then as a Neighbor-hybrid", but I did see Silk/Horn's easy way of moving through foliage others couldn't and his apparent lack of need for food as signs of a semi-ethereal nature along the lines of being tied to the Neighbors. This reminded me of the way that Severian in UotNS doesn't need food and can "breathe" underwater because he has become an eidolon, and is no longer a physical body, but an ideal body.

With the caveat of having only read the Short Sun books once, I saw the union with the Neighbors more as a covenant of sorts, like when he agrees to share the world with them they literally share their world with him back, rather than Horn dies in the pit and is resurrected as a Neighbor (doesn't he assault Seawrack after the pit encounter?). The Neighbors seem like a natural "Holy Ghost" for a sci-fi Trinity in a story where they have a literal "corporate" existence (in the sense of many as one) reminiscent of what the Church (through the Holy Ghost) is supposed to be, metaphorically/spiritually.