To make things easy to use, an automatic switch that can accept two RCA or 3.5mm inputs to one output would seem to be the answer. I can find HDMI auto switches, but not ones for analogue audio.
The amp will come out of standby when a source is playing, and go into standby again if the signal stops, but only on the input that was last selected.
I've had that a few times but never got around to reporting it. As you say changing some of the text (absolutely nothing offensive or remotely dodgy involved, no embedded control codes) will allow the post to go through. It's a weird one for sure.