Misfire problem - P0351

[jtg575]

Thought I'd see if anyone else has run into this issue. I've got a 2010 Vibe (Matrix) base, 1.8L auto. Several days ago:
- No signs of any issues, very smooth running, until one morning, out of the blue, it wouldn't start (turns over, brief stumble, then just turning over with no sign of ignition). Didn't have time to look further that day.
- Next day, after some stumbling, it started, ran rough briefly, then smoothed right out and purred. Didn't drive it.
That evening, started fine again but immediately ran rough and threw a misfire code (P0302, cylinder 2)
- Pulled all plugs (NGK Ruthenium, about 50,000 km). All were fine (including cyl 2) except cylinder 1, which was actually loose: still threaded into the head, but I was able to turn it out with my fingers only. Lots of soot around the coil boot. Turns out the ground electrode on the plug had broken off, maybe from the vibration of not being tightly seated in the head.
- I ran a scope down the head into the cylinder to see if I could see the electrode bit; fished around in there with a magnet, then with a vacuum, blew a bit of compressed air in, tried again, nothing. I started it briefly with the plug and coil out to see if the piece would get blown out, but nothing. Rechecked with scope, magnet, vacuum. Nothing.
- Put in a new plug, reinstalled original cyl 1 coil, ran it for a few minutes.
- Running rough, computer consistent misfires on cylinder 1 (though no P0301 code), and also a P0351 (Ignition Coil 'A' Primary/Secondary Circuit).
- Swapped coil 1 and coil 4 to see if misfire moved to cylinder 4. It didn't. Still misfiring on cylinder 1, which has both a new plug and a coil that was working fine previously on cyl 4.

Any thoughts on where to go? I could, of course, replace all the other plugs, replace all coils and try again, but it seems strange to me that the misfire stays on cyl 1 with the coil swap. Maybe that bit of electrode is jamming a valve or something?