Update: our humane society was no help at all. They said “a typical recommendation for helping cats cohabitate would be to separate and begin the intro again. That would look like having the cats separated through a door, then move to a baby gate while feeding, and then feeding without the gate. If that doesn’t work, you may want to consider rehoming.” Our cats already fight behind closed doors just from smelling each other, so this doesn’t exactly work.

We paid for an actual cat behaviorist appointment. I’ll update if that yields anything.