There may be some interference with energizing the frame with a method like this, especially with 5 volt pixels?
'Shouldn't. It's not much different than your car, you know. A low capacitor across the axle halves or the hubs, as the case may be, should get rid of any HF across the power leads.
You might create problems for your wireless signals
If the spokes are metal, then the wheel could act as a Faraday cage, but at 2.4GHz, I don't expect too many problems. One might have to be judicious about positioning the Pi. IF the whole thing except for the fasteners, the hub, and the axle are plastic, then the question is moot.
Even so you still will need some form of slip ring arrangement to get power to the frame and controller on the wheel.
No, one does not. That is the whole point. The metal bearings will carry the power between the hub and the axle on each side. If the bearing is fitted into the hubs themselves, then the internal leads are attached to the hubs and the external power is attached to the axle directly. If the axle is attached directly to the hubs, then the bearing mounts will be attached to the axle support, and the external power is attached to the bearing mounts, with the internal leads attached directly to the axle. Either way, power will be transferred to or from the bearing mounts to or from the axle. No slip rings required, and while ball or needle roller bearings are not absolutely required, they are the best engineering solution to mounting the wheel, and given they are already planned, add nothingto the cost for transferring power.