All the 4 HDMI ports support HDR,The first HDMI port is an ARC port for soundbar connection and Video passthrough. To activate HDR , u just need to activate it after you connect your device. Press menu on your remote, select input settings , pick the HDMI port you are connected to and turn on Full UHD color . Its that simple
#4 Is the Youtube TV app loaded in or do you have to Chromecast it with another device?
There is a built in YouTube TV application on the SmartCast input.