OOOOH this is one of my favorite theories but I hadn't considered BONE as part of the ceramics, that just makes it even better!! š
The Anastasia bit is also like, yeah almost tangential to the general Cassiopeia theory, but everything in this series is done for a reason so the note from Anastasia in the Sixth lab feels important (and the note at the end of Dr. Sex could have easily been from her?), and liiiike. Team 69 through the ages come onnnn. xD And it definitely wouldn't hurt the whole "Cassy played long games" thing to have worked with Miss Mysterious Pact With Alecto, eheh.
But yeah more potential stuff:
Cassy died specifically to Varun, who we've now met. And who ... doesn't actually seem all that murderous. It doesn't attack until Nona calls for help, and then it says "I came to help! I am made a mockery" and "Their vengeance is not my vengeance."
In fact, to this day, the only Lyctor that ever even might have been directly killed by a Resurrection Beast (assuming you don't count Harrow or John as Beasts) is G1deon, six months ago, and his death is mysterious as hell. Why did Varun run? How did Pyrrha walk away? (Especially an interesting question given "Camilla and Pyrrha didn't answer" above, implying Pyrrha's silence is also significant rather than only Cam knowing.) What ever happened to Nonius and the cav crew?
The rest died by their own plans. Cyrus drew part of one into a black hole. Ulysses dragged one into the stoma. And Cassy was torn apart by feral ghosts ... though Augustine says "disappeared into a large intestine" and then got ripped apart? Don't ghosts usually flee from RBs? Is Varun carrying some around? Or I suppose the ghosts could be a matter of RBs always looking different, eldritch in nature and all, even when two people look at the same one at the same time. Maybe that's only what it looked like. Either way, if we trust Augustine's account, Varun might have eaten Cassy, in a way, which ... I guess may or may not count as directly killing her / directly contributing? But if we trust Mercy's account too, it sounds like at most it would be indirect, and either way, this was Cassy's plan to drag it somewhere just like the first two deaths.
So RBs in general don't actually seem like they're trying to kill John or the Lyctors, but chasing them for another reason. (Maybe even ... much like John's original intention for the trillionaires, wanting to drag him back to the system to fix what he broke? Which in turn might kill him, literally or in essence, or otherwise activate whatever the reason is he refuses to cross that threshold even a little. After Canaan House he's "nearly in Dominicus's halo", but won't cross. But still looking to hold him accountable and maybe help Alecto rather than just take vengeance. š¤) And then we have specifically the same RB between Cassy's death and G1deon's mysterious circumstances + the cav crew.
So I have to wonder if Cassy somehow made a deal with Varun way back when...
And ALSO. Way more debatable, but the first time Harrow travels through the River, in HtN Ch7, the chapter skull is the Sixth and they do talk about Cassy quite a bit, and maybe that's all it is.
But at the end, when Harrow sees the five points of light and wonders "...five?" I... don't think it should actually be Gideon, or Alecto, because I think if one showed up as a unique signature, both should. Plus Mercy and Ianthe only read as one signature rather than two, Harrow is vaguely aware of herself as a sigil with her 200 but still reads as one big light, John's only one big light. Gideon and Alecto should both just read as another part of Harrow (and/or John).
I don't think it's Wake in the sword because they're all described as incandescence, and I don't think she would stand out from every other normal ghost in the River + if Harrow could perceive her surely John could and would have caught her right away.
And then perhaps most importantly: it specifies that one light was below them. There's no cargo area on the shuttle, it's this one little room and a tiny probably-bathroom on one side, still not below. Below makes it sound like something outside the shuttle. So... I wonder if, somehow, for some reason, that light could have been Cassy... Why? I don't know but I'd sure like to...
I need to properly meet this woman so badly.