actually, seems like it was none of the above. I think the root of the issue here is the tolerance stack between the (Screw-Split washer-rod end-collar-axle slot) system. Essentially, the screw is not really the right length to go through all the aforementioned. things and then properly interface with the slot, so no matter how hard it’s tightened, it never really does anything other than clamp the rod end to the collar.
To test this theory I removed the split washer from the stack, in effect substantially lengthening the screw so that it can definitely reach the bottom of the axle’s slot/flat but will no longer properly clamp the rod end. Here I only need to slightly tighten down the screw to rigidly rotationally connect the collar to the axle. notice when i try to twist the collar, it can not move without also rotating the axle and everything else on it.
and then here’s what it looks like normally (with the washer installed as i received the trucks). notice how the collar still has some freedom to move around despite being clamped much harder than previously.
In both these cases, the collar’s normal clamping screw is fully loosened.