I don't think that this setup will work, but I would also think it's not even necessary.
As far as I know the AppleTV has a regular HDMI
output. Audio and video go to the receiving device's HDMI
input (usually a TV). The audio signal contained in the Apple TV's output cannot be send to the Ultra's HDMI port, which technically is an HDMI
output, sending no video, but expecting an Audio Return Channel signal. That signal is different from regular audio send to HDMI inputs.
Adding a splitter wouldn't change that. I have seen splitters that can pass through the ARC signal, but none that would be able to create it.
But if your TV has at least one regular HDMI
input to connect the Apple TV to and another HDMI
input supporting ARC (not eARC) then you could hook up the WiiM Ultra to this HDMI input labelled ARC. The TV receives audio and video from all kinds of sources and one of its HDMI inputs is able to send audio data back to the connected device. It doesn't care if this device is really sending any video or not. That's the entire idea behind HDMI ARC.
So, if I understood your setup and your goal correctly, the setup would be:
- Apple TV HDMI out to TV 1st HDMI in.
- TV 2nd HDMI in (marked ARC) to Ultra HDMI port.
- Ultra analogue RCA out to Kenwood RCA in.
The obvious drawback is that the TV needs to be powered on to pass whatever it receives from the Apple TV (e.g. AirPlay) to the Ultra.