Will Apple TV 4K work with legacy 1080p HD home theater system?

I currently have fiber optic AT&T Uverse service feeding into an AT&T residential gateway (Arris Model BGW210) which feeds into an AT&T STB/DVR (Motorola Model VIP2250) which feeds into a legacy 1080p HD home theater system. The home theater system centers around a 5.2 channel audio-video receiver (Marantz Model SR6006) which feeds into (a) a 1080p HD plasma TV (Panasonic Model TC-P55VT30) and (b) a 5.1 channel in-ceiling speaker system. (Note: The home theater system supports 1080p HD via an older HDMI cable, but does not support 4K UHD. At some later date I hope to upgrade the TV and AVR, but not now. A blue-ray disc player also feeds into the Marantz AVR.)

I hope to buy an Apple TV 4k (5th Gen.) to stream video from existing Amazon Prime Video and Netflix accounts plus play music from an existing iTunes account. The logic behind getting the more expensive Apple TV 4K over an Amazon Fire TV Cube or Roku Ultra is the desire for better integration with existing iPhones and iPads in an otherwise PC household.

I plan to run a CAT-6 ethernet cable from the existing Residential Gateway to the Apple TV 4K box, and feed both video and audio from the ATV box to an auxiliary input on the Marantz AVR via an HDMI cable. The Marantz AVR would then sent the video to the TV and play the audio over the 5.1 sound system.

Questions:

#1 - Will the Apple TV 4k send the appropriate 1080p HD video and audio signals to my legacy home theater system to accomplish 1080p HD video streaming?

#2 - Since I am only trying to achieve 1080p HD video streaming can I use an older HDMI cable (i.e., not a 18GB cable)?

#3 - Will I be able to play music from my iTunes account over this setup?

#4 - Any potential surprises that I should know?

Thanks in advance for any help.