Just spent a couple of hours reading through this thread. Frankly I'm amazed you're pulling ~2MOA from cast lead 22 cal bullets at 2800+ FPS, ESPECIALLY using ACWW alloy.
In my own .22 cal tests through my AR thus far, the superiority of powder coating vs lube for my own purposes has showed up easily on paper. My best results so far have come from bullets that were cast, sized/checked, powder coated, and then oven treated and quenched for an age-hardened BHN around 35. My problem is definitely fit; I'm using the Lee 55 grain FN and I think there's just all sorts of room around the bullet in my chamber, there's no way it's going into the bore straight and concentric. But at least I'm not getting any leading.
Full Lead Taco is a great guy, turns out we live really close to each other, and he gave me some really hard foundry type ingots to try hardening up my .223 AR bullets. I haven't had to try that yet, but it's nice to have the option if I can't solve my accuracy issues by using a better mold.
I also can't seem to get decent results without standing my bullets up, not touching each other. I ended up actually CADing up and 3D printing a tray where I can put my bullets, nose-down after they come out of the shake 'n bake tub. Then I put the metal tray for the oven on top of the 3D printed tray, flip the whole thing over in one smooth motion, and carefully remove the plastic tray. Voila, 150 bullets spaced perfectly and standing on their bases. I do the same thing for my 9mm pistol bullets, but I just use empty commercial ammo trays instead of a custom 3D printed one.
Looking forward to seeing what else you can do here Ian, but honestly it kinda seems like maybe your problem is the gun. You fixed the bedding and harmonics stuff, but I'm guessing your barrel needs some love if it fouls that bad with copper. Can't wait to see if things tighten up after you get some fire lapping bullets downrange.