So, after head-scratching, a couple days in the garage, and lots of trial and error, I got the P0410 and all my misfire codes cleared! Not even sure the combination of things that led to that... but it involved the 90º elbow on the airbox side of that vaccuum line, having a crack in it. I used superglue to fill the crack.

3 days of no codes and suddenly my check engine light came on. I took a look at that vaccuum connection and sure enough, the superglue let go. Turns out Advance Auto Parts carries a universal elbow with the same measurements for $3, so I did that.

Another 2 days, no check engine light. Figured I was in the clear. Just for the hell of it, I plugged in my code reader.

Another new code! This time P0455 - Evaporative Emission Control System Leak Detected (no purge flow or large leak)

It seems no matter what, I'm gonna have a code...