#1, join a different forum, this one is geared toward installing CarPC's, but you did find a good place to post!
#2, I can answer that one though, because I love home audio. Your speakers need power. They have to be "amplified". An RCA or Optical output is not an amplified output, hence the need for a receiver. They sell High to Low because the information is there and can be stripped for RCA use, but to do the opposite you need to add power. I would research a "Class T Amplifier." They are generally very small amp/preamp units that can be used for that purpose, assuming you don't need a ton of power for those outdoor speakers. Parts Express usually has a bunch.