Assuming I intend to use cable chains on my Subaru Outback 2017, can I also assume (if clearance is ok) they should be used on the front tires vs. rear?

forget it, they break, they slap the fenders, just get some studded tires or blizzaks on that light car you'll go anywhere

Not familiar with the ebay plastic solution. I am however a fan of snow tires. I didn't invest in the more expensive tires but got decent snow tires and they've lasted 3+ seasons and still have a ton of life left. Never had an issue getting anywhere when needed.

Whatever you decide to try, treat all four corners equally, as the AWD transfer case really needs to "see" equal rolling radii...especially at speed.

Subaru recommends using type S cables on the front only. It is in the owners manual. I can't see why you could not use them on the rears unless there is no clearance for the ABS cable. Why do you need cables? AWD and good tires should be good enough in almost all situations. Just carry cables for emergencies.

In extreme conditions, Cali HP may require cables or chains on AWD. Or they may check that you have them availalbe

You should always carry chains in California snow country or you may be turned back by the CHP.