When your learning by doing new stuff it is all good
Test, test and keep testing until you exceed the speed of light

Even old gas could have caused it depending on how long it was sitting. Fuel loses octane over time, if it sat for months and you took it out and beat on it you could have rattled it pretty good. But it looks like it was probably knocking for a while. How much timing were you running?

The borescope is a liar! Towed it to the shop and took the turbo and head off and found it to not be all detonated up like I was expecting:

The burn looking marks at the top of the bore, what is that?

Burnt oil making coke and carbon sticking to the cylinders It should scrape off with a sharp blade
I would take a thin bladed screw driver to try to see if all the rings on every piston are loose or not, bring it up to TDC and check both pistons at TDC to see if you can rock the pistons in the bores by pushing on top of the outside edges of the pistons directly above the rings out near the edge of the block and then again on the inside of the piston near the cam, if the pistons move freely in the bores your good, if they don't take them out to find out why they won't rock in the bores easily
Let us know what you find, how did the leak down and compression tests go?

Compression was 118-122 for this side.

It's 4hrs away from me now, so it'll be awhile before I get back to rock the pistons and tear down the other side.

If the other side is good, I've got some better ported heads I'll through back on it when I put it together.