I just read thru all 16 pages.. what a ride.
man, I wish you'd post up all the used parts you took off forsale. they are probably in better shape than the new parts we are buying at auto parts stores lol.
the tune is interesting. instead of searching for better fuel can you just ask them to pull a deg of timing out of steady state cruise area of the map? that's what I did to my ls3. is basically your engine without the cam timing controls a different stock cam in a lighter manual car, but even so I pulled 1 deg out of the entire stock spark map besides idle and decel area. I didn't log knock as well as you, but I saw some noise and engine just felt raspy for lack of a better word under acceleration. so after removing a deg it feels smoother thru the whole Rev range. I have a feeling in a heavier truck with a auto that's always trying to keep rpms low on the interstate they are just asking to much of our weak 93 these days.
have you done anything to alter the intake filter and box setup? you may log maf hz and graph them for sport. I found anything besides a completely stock airbox and oem filter, the maf signal got *noisy* for lack of a better term again, on the interstate speeds, just slightly reduced throttle in that odd area where that throttle isn't closed enough to get into decel engine braking but less than steady state cruise amount. being a manual it caused a slight bucking and drive line slack the ecm didn't like at all. I couldn't find a way to tune around it that I was happy with, tried 2 different air intake systems, a k&n and a corsa. both were bad, eventually I gave up and bought a used oem box and all was perfect signal wise again.
now this was not one of our trucks, as i have a totally different from yours 6.0L in that with Cathedral Port heads that lives on 87 but a manual car, the auto in these trucks might hide any of that feeling while driving, except from a log you're seeing. but I mention it only because you seem very detailed in your troubleshooting and logging data.
very nice caddy. I wish I had have your motivation to fix my yukon up but the expense and lack of quality parts over the years has led me to more of the if it ain't broke, don't fix it till it is broke mindset with preventive maintenance.