Hi, cannonball2! Sorry for my late response to your comments (thank you!)
First, I have to agree about "Properly done the old rag joints not to bad." Look at the numbers (no, I don't actually have the numbers, but.....). I have to believe that probably 99% of the tens of thousands of motorized bikes use the rag joint. Yes, I would love to use a sprocket adapter instead, but they are simply cost prohibitive for me, whether for a build for my son and me, or for re-sale. 50-80 percent of the cost of the complete engine kit!
Interesting about having added plates under the dished sprocket. Did the plates contact the spokes? I would be concerned about that...concerned that the metal would cut into /cut / weaken the spokes.
Yes re loading the spokes excessively ... hence my "how tight is tight enough" question. Too funny saying "too little and it will spin and remove the spokes for you".
I have not yet had a build where the sprocket bottomed on the hub..which, at least in theory, would seem to help lateral ("wobble") alignment...but it would also limit the force of the rubber pads on the spokes - which as you discuss, must be tight enough to fully "grab" the spokes. FYI, I use a dial gauge to check the lateral runout... the best I can consistently achieve is about 0.020 inches...roughly 0.5 mm. I am convinced that the sprockets that come with the China Girl kits are not completely flat... that is, they have their own "built in" wobble /bend. Grrrr I also suspect that the toothed circumference of some or even most is not a true circle (meaning having a slightly flatter area) which makes it impossible to maintain consistent play in the chain (while rotating the wheel and sprocket) while setting the tensioner. I did use a spring loaded tensioner (the one sold by turbobuick1 on Ebay) on a build for personal use. The tensioner is a beautifully engineered and manufactured piece, and works great - despite my concerns about the top / drive side of the chain loosening when starting or backing off the throttle ... but, again, using this adds just over $40 to the cost of the build.
Good advice (thanks!) about checking the tightness of the rag joint on regular intervals as maintenance, and especially a few times after the initial install...as the rubber compresses where the spokes contact it.