MacBook Air 2020 MacOs Sequoia v15

[Rex255]

I have a MBP (M1 Pro) running Sequoia 15.3.2.
I was using a WUX500 via HDMI in extended desktop, all working fine.
I did something and the WUX500 went blank and I could not see it in the OS.
I tried my wife's MacBook Air and it worked fine in extended desk top. I did something and the WUX500 became invisible to the OS.
Support calls over four days Apple eventually decided that I should erase the HD on the MBP and start again. That worked.
A few days later, just before a big seminar where I was in control of the projector, it was all working until I changed from extended desk top to mirrored desk top. I then had a laptop that could not see the WUX500, again.
Apple support now say it is the problem with the firmware or driver for the WUX500.
Canon in a preliminary call say it isn't but have raised it with their engineering team for further consideration.

Interestingly Keynote does recognise the WUX500 and allows it to be used but doesn't allow colour management which for a photographic seminar is useless.

Apple support also instructed me how to rebuild my MBP but I think they did it wrong as I and Apple support cannot access Time Machine backups going back to 2021. We can see them but access is denied.

Any ideas very welcome on the WUX500 problem