I don't think I can send it via the amp, or at least I don't think I'll be able to once I upgrade my monitor. It is an oldish amp with old HDMI that won't be able to pass through higher refresh rates. At least I think that is the case.

I didn't think mirroring the display would be a good plan for similar reasons, I'm not clear what it does to the refresh rate on my main display it is has to mirror to an HDMI port that can't get to the same refresh rates. That is something I'm going to investigate though.

Stopping the mouse from getting to that display seems like a nice practical solution. I'll bet there is a tool out there already to do that.

I've just experimented with organising the displays as you suggested and that helps a lot, though it does make it slightly harder to get back onto the main display when you do manage to mouse off

Optical/coax won't help, the whole reason for using HDMI was to get uncompressed 5.1 audio out. Optical/coax can only do two channels of uncompressed audio. And yeah, I'll never hear the difference, but I just wanted to see if it would work...

As you say, ARC won't help me with this one.