Yup, my own thinking hasn't changed. TBH, I'm not the type who would motorize a Walmart bike to begin with, and I think I still could have made 14g spokes work with a sheave in your case. I ran a ragmount for 4 years and well over 10K miles, as well as a kit idler, and never had problems with either. Never once even burned out a chinagirl clutch. I've owned old stuff that I had to fix myself my whole life, so I tend to be careful with it.
As I've said tho, I've never run a sheave, so me making work what you didn't is just conjecture on my part. As far as belt drives, I have to admit I've never run a full-belt-drive bike. I have a Honda-style 4G, which is belt-drive and been on the same belt for 7 years and ~10K miles, but it's on a system that also has 3 additional chains.
I don't recommend steel rims, not even on a pedal bike. They are heavy, prone to bend, and will rust inside out. They totally suck with rim brakes if you happen to run them. If you want a strong wheel, get a double-wall downhill-rated aluminum wheel, whatever hub you're gonna use, and have it built with some 12g Sapim spokes if you want serious business. It's what I did and I had em laced 4-cross. 4-cross may not fit your sheave's mounting points tho.