A:Answer I will start with the second question first. The HDMI output is intended to be connected to the HDMI-ARC input of a TV. With the sound bar connected in this way, my Samsung smart TV automatically senses the sound bar and turns it on and off with the TV. It also allows the TV remote to control the volume on the sound bar. During my experimenting to find the best setup for my needs, I did connect my BluRay player (Sony) to the sound bar and then the HDMI out from the sound bar to a non-ARC enabled input on my TV. The signal from the BluRay player was passed through without issue, so I suspect that you could use it to feed your surround sound amplifier though it was surely not designed for that purpose.
With regard to Dolby Digital Plus, the manual indicates it will decode this and I can say that I have had no issues using Amazon Video, Youtube, or Vudu from my TV or BluRay player.
For reference, my final setup has my Sony BluRay player connected to the sound bar's HDMI input and the HDMI output connected to the HDMI-ARC enabled input of the TV.