#5 Exhaust rocker off the valve
#5 Exhaust pushrod bent
#5 guide plate broken
SB gets the golden ear trophy

Pics coming momentarily

Nope. That was #7. #5 had only 5% leakage.

Yep.

The top of that valve took a hard hit and now may be bent and hung open as well.

that doesn't explain what killed the plug, there's some debris loose in that hole,

That's what she said.
Joking aside, a piece of ring land maybe.

I just borrowed a cheap usb borescope from a buddy, didn't really see much. saw some clean areas of the piston at the outer periphery, possibly consistent with the rich burn that was occurring.

I guess my big question at this point: Was this a coincidence, happening right as I began to feed a tiny amount of water in the intake, or did I cause this? And if I caused it, how?

Leaky header syndrome is possible......water don't compress. Pressure check your header with attention to that cylinder to be sure.

Add: Looking at your pics I see a clean area in that pipe that appears to be a water leak although looking at a distant pic makes it difficult.

There is a small amount of rust in the exhaust ports of cyls 5 and 7 in that header. Not a lot, but there is some. There also was a little bit of water in the bottom of the V-band clamp at the header-tailpipe junction.