You should be able to adjust the width of bolt on hubs somewhat with spacers if necessary.

I would not use the Off Center/OC/Asymmetric rims for a single speed. It just isn't necessary. I've used the Velocity Aerohead, and liked it. The A23 should be a good rim. Note, of course, all current versions of the A23 are "tubeless ready". It means the tire has to be forced into the center channel to mount. Other rims may be easier to change tires with.

There are several coaster brake multi-speed internal gear hubs. They will increase the weight somewhat, but should be considered.