Are you sure? It's hard to imagine that a 4K television would not support ARC.
Agreed, because the receiver does not support ARC (it's the same as the Yamaha RX-V1900 which is HDMI profile 1.3)
Unless OP is using the SmartTV part of the television.
Unless one of the sources is 4K.
Agreed, but it's often the most efficient way to stream 4K content (assuming the television supports it, I didn't look into that).
You aren't kidding there. If the OP truly wants that to be the case he needs to upgrade the AVR.
An HDMI audio return was not mentioned in the spec sheet of the TV or the receiver. If it is present is is usually touted in the spec sheet. I admit I did not download the manuals.
If you are going to use 4K from the TV, you will have second class audio. HDMI return only supports two channel. Digital Toslink does not support the loss less codecs.
4K as far as I'm concerned is a non issue. At most listening distances, including my theater, you would not tell 2K from 4K.
The next issue is that NONE of these 4K screens are nearly as good as my high end 2K Panasonic plasmas. I continue to look at screens every chance I get, and even the most expensive do not come close in overall quality to the two Panasonic screens my wife and I enjoy.
4K is just market driven nonsense. What the industry needs to do is going back to getting screens as good as they used to be, and then may be worry about 4K.
Trouble is it won't be long before people forget how phenomenally good those older screens were.
4K is absolutely not worth poorer audio, and not likely worth lick of spittle on current screens.