A:AnswerThe spec on the 4K fire stick audio says it supports "Dolby Atmos, 7.1 surround sound, 2-channel stereo, and HDMI audio pass through up to 5.1. Dolby Atmos is not currently supported on Netflix titles." -- it should work just fine. You should be able to set that input as 5.1, 7.1, or 5.1.2 without a problem.
A:AnswerThat depends on what you mean by simultaneously. If you mean as any other input mapped to an output, yes. If you mean as an overlay on top of another signal karaoke style, no. It will only send one input to a given output at any time.
A:AnswerYou can adjust relative volume, maximum volume, startup volume, etc., levels under setup (3-5). Also can adjust volume difference for different source volume under Intellivolume 4-1.
A:AnswerIt would work fine for that application. The question you should be asking is where the cell phone will be relative to the receiver. I haven't been to impressed with the bluetooth connection. The connection will be in and out when I am only 15ft away with direct line of site.
A:AnswerI had a similar issue a couple different times. Once the hdmi cable went bad. The other was the actual plug-in for the hdmi on the back of the TV had gone bad. Hope this helps.