Ok, so the fact that it seems like Nona is Alecto in Harrow’s body seems to complicate the “Anastasia is The Body” theory but I actually think I’m getting it here; the theory is rooted very solidly in textual evidence and everything we’re learning just refines some of the details slightly. Either:
1) Alecto is in the Locked Tomb on the Ninth but the text is filled incessant references to a tomb on the First because like, THE Tomb is in Canaan house, in the sense that it is THE tomb precisely *because* it is empty; it is where Alecto, the Earth resurrected, walked out of. In a way the entirety of the Earth is the tomb, but I do believe that there is literally a room or space in Canaan house that functions as this tomb and returning there will let Harrow and Gideon find vital knowledge about necromancy/resurrection/John/the end of the world
OR, more likely, my personal theory:
2) Alecto was “shut off” and her body entombed on the First, but immediately her spirit or soul attached itself to Anastasia. Alecto communicates to her by visions and voices, and Anastasia becomes a saint in a very old sense. She is possessed of a terrible clarity her colleagues lack and possessed by a deep rage against God, and she begins her mission to somehow end him in secret. Either John really does want to mislead others by building the locked tomb or something tips him off that Alecto lives in Anastasia, or perhaps both. Either way, when the time comes to seal it he seals Anastasia inside.
So the body in the tomb is indeed Anastasia’s but The Body, the ghost entity that haunts Harrow, is Alecto. It appears to Harrow as Anastasia’s corpse, possibly on purpose because she understands Harrow loves it or possibly because that is just how Harrow’s brain interprets her. We have seen The Body appear to Harrow in different guises and she speaks in others’ voices, so the possibility that her main form is not actually hers isn’t a stretch. Anastasia’s soul is either long gone or has somehow been absorbed by Alecto (possible, given that Alecto is already billions of people and every other living thing on earth). John’s wards sealed Alecto in the tomb (the text makes much of ghost wards!), preventing her from finding another host(?) until Harrow came along.
At the end of the latest excerpt John addresses the person he’s speaking to as Harrowhark even though everything else indicates that it must be Alecto. In a literal sense I believe this is because Harrow is experiencing Alecto’s memories in a dreamlike fashion, but on another level this tells us that Alecto is wearing the guise of Harrow (in the sense that she is currently operating Harrow’s physical body and in the sense that she once wore the guise of Anastasia). The name Anastasia means “resurrection,” and she WAS Alecto’s second resurrection. Harrow will be her third and final resurrection.
Either way, a tomb lies empty on the First (either entirely empty or empty because Alecto is not in whatever body resides there), and we’re gonna go in there, and we’re gonna see and believe.