Well all I can say is regardless of all the number crunching and potential "smoke and mirrors" stuff I sure would never recommend or consider using a spring or WDH bar rated less than the expected TW and in fact am in the camp of using one even a couple of hundred lbs heavier than the actual tongue wt. If this causes issues then I think it's the trailer with the issue and not the spring bar size selection.
I'm pretty sure at least Equal-i-zer approves of this approach and I would put my money on their analysis and engineers over anything that might be posted here. I admit I haven't tried to read and understand every word in every post in this thread, but would be VERY SURPRISED if any of the like two or three MAJOR WDH Manufacturers (Reese, Equal-i-zer, EZ-Lift) would recommend using a spring or WDH bar of significantly less rating that what the actual TW is and certainly not approaching the 1/2 figure being talked about.
Larry