Interesting ideas, but I think it must have something to do with the accessories (fender, trailer). I've had 2000+ trouble-free miles without these bits installed. Needless to say, I'll be leaving them off indefinitely. I'm pretty convinced that horizontal dropouts with QRs are simply not up to the task. I don't expect further issues.

FWIW, I'm running Dura Ace 7400 hubs with the original skewers. I just checked that the locknuts are properly installed and that the axle length is even. I also double checked frame and dropout alignment, since I did spread this frame.