You'll most likely discover they value location more than the actual roost used (up to a point... they're not going to want a 1" metal rod): https://www.backyardchickens.com/articles/the-perch-experiment.74272/
Rounded off handrails are IMO probably close to the ideal shape and size bars for...
19 gauge should be fine in most applications, however if you can afford 16 or 14 that would be thicker wire that should offer more protection. For the predators you have listed: Coyotes, opposums, skunks, fox, raccoons, hawks,falcons, bald eagles, etc.... 19 should suffice as long as it's well...
Not necessarily: https://www.backyardchickens.com/threads/how-cold-is-too-cold-check-this-out.1345546/ Not saying you should deliberately push the envelope, but healthy adult birds with good shelter can survive some amazing temperatures.
The handrail looks like a good size and shape for chickens.
I'm not a fan of heaters but if it's for occasional use like a sudden drop to -30 degree (without windchill) for several days then I don't think that's necessarily a bad idea. The chickens would probably be fine without it, but I get...