It'* strange that it doesn't pick up knock when you can hear it but does once you let off the gas. It'* especially strange because assuming your timing is not insanely advanced (which it looks normal) with a wicked lean condition (which also looks normal), detonation only comes under load. Even then you're more likely to see detonations
evil(er) brother, pre-ignition.
A knock sensor listens for a specific tone. If it'* given that everything is in excellent shape from the PCM to the knock sensor (and the knock sensor isn't too tight or loose), I suspect you may have something rattling at just the right frequency to set it off. It'* possible that while under load you can hear it better but free revving creates that certain tone. Some SII guys use a sound barrier on their transmission and around the knock sensor for just that kind of issue.
Maybe try revving it in neutral (don't go too far!) and see if you get a few degrees of KR that way. Another way would be to take it up to speed and manually downshift it. You could take that a step further and tap the gas a few times while slowing down just to jolt the car. If you get KR with either of those you should spend some time checking that everything is bolted down tight and nothing is banging against the block/trans. If you don't get any knock then definitely try the stock pulley and see if that hushes it up.
Just a thought..
Stuff got done.