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Note: Some people solder joiners on a curve. Others would debate whether doing this on every joiner wise given expansion/contraction issues. I’m not experienced enough with doing this to comment on the wisdom of this approach. I will only address the power feeder question.
A good rule is that every piece of track should be soldered to something; whether it be the next piece of track or a feeder. Since you are going to solder every joiner in your helix, it should be adequate to run a bus up one side of the helix and also half way around the helix. Definitely do not run the bus completely under the spiral of the helix.
I was in a club that supported their helix with threaded rod. Then there nuts to support each spiral of the helix. This was simple and made it easy to get the incline of the helix just right. They applied power to the threaded rods. The threaded rods became the bus. Then wires were run from the threaded rod to the track. The feeders were attached to the threaded rod using star washers that have solder tabs on them. You should be able to get threaded rods from Mouser, Digikey, or similar electronics supply stores.
Wiring For DCC