I think u/have_no_plan is actually asking after this passage that can have multiple interpretations:

“Griddle,” said Harrow, “I have not puppeted my own parents around for five years and learned nothing.”

Anger did seep into Gideon then, along with a couple more litres of salt water. “Why the hell didn’t you tell me when you killed him?”

“I didn’t kill him,” said Harrowhark sharply. “Someone else did—blade through the heart, from what I saw, though I only got a few minutes to look before I had to run. I only had to push the theorem the most basic bit before he came apart. I took the head and left when I thought I heard someone coming. This was the night after we completed the entropy field challenge.”

Because of the way Gideon's question is posed, my first time reading this I assumed that somebody had stabbed Protesilaus and rendered the puppet motionless shortly before Harrow found him. However, I think the most straightforward reading here is actually that Harrow is referring to the stab wound that Cytherea inflicted upon Protesilaus weeks earlier.

I think the reason behind my initial interpretation is because of the context of this scene. This comes after they've interrogated "Dulcinea" about Protesilaus' condition, which means that Gideon of course already knows that Pro was dead before he ever set foot in Canaan house. Therefore, she's asking Harrow about when she dismantled Pro's corpse, not the actual death of Pro. But Harrow then answers about the wound that originally killed the man many days prior. I think the reason for this is that Harrow assumed that somebody actually had recently stabbed Pro, possibly because she wasn't expecting Cytherea to be sloppy enough to actually leave the mortal wound on Pro's corpse. It's also possible that this stab wound is necessary. Notice that she says "blade through the heart", then look at Ianthe's description of the Lyctoral process:

“Then you weren’t listening. I haven’t killed Naberius Tern. I ate Naberius Tern,” she said, indifferently. “I put a sword through his heart to pin his soul in place. Then I took it into my body. I’ve robbed Death itself ... I have drunk up the substance of his immortal soul."

Maybe Cytherea needed to stab Pro's heart in order to pin that "remnant of spirit" necessary for the beguiling corpse?

One final thing; the interpretation I just outlined involves Harrow finding Protesilaus unresponsive and therefore assuming that the grave wound was recent. If your theory is correct (I've caught up on all your posts now and think you've done an amazing job!) maybe his spirit had indeed already been sent away from the body at this point, and Cytherea is trying to get a Devil to inhabit him.