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

Pics coming momentarily

https://cimg0.ibsrv.net/gimg/www.off...f4bd3b5467.jpg
https://cimg1.ibsrv.net/gimg/www.off...b462cf48ee.jpg
https://cimg8.ibsrv.net/gimg/www.off...cedc42a9be.jpg
https://cimg1.ibsrv.net/gimg/www.off...db2f757bfd.jpg
https://cimg3.ibsrv.net/gimg/www.off...6eb5f0635d.jpg
https://cimg6.ibsrv.net/gimg/www.off...2132c08fea.jpg

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

Yep. :kiss:

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. :cool-smiley-011:
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.