well I read every word here, and I can say that I have no experience with the engine you are looking to use, I don't have any of the fancy hubs you mentioned and I don't have a bike with a shift kit, but what I do have are three very dependable bikes.
The fact that you are wanting to do such a complete ground up build, I'm hoping you have a good selection of tools and plenty of mechanical experience to get it all done right, personally if all I ever intended to go was 15-20mph Id just order a friction drive setup from Dax and be on the road in a couple ours and just honestly keep it that simple.
with what you are wanting to do, you should have a nice dependable bike, but most of these little engines build there power under the higher RPM range so if I were wanting to just put around at 15-20mph i would put as big of a sprocket on the back wheel as I could, but then again as I said earlier, I have never used the little 50cc 4 smoker engines so I really am not qualified to tell you 100% what would be the best set up for what your wanting to do, all I know is that there would be no way I would spend all that money on a bike to only go 15-20mph... but hey just like it has been said in the past here.. " it's your bike, build it the way you want it"
sorry I cant help you with the questions you ask, but I just wanted to ring in here and let you know someone is reading the post and at least has some sort of opinion regarding the build plan you have laid out.
best wishes and I hope you can get it to where it just what you want it to be.