I don't think there is any doubt that they could cut away in vision leaving network audio or vice versa. Setting up a gallery that couldn't do that would be a challenge.
Yes - and sorry if it sounds like I’m duplicating earlier discussion - but I can’t think of any reason they would ever need to opt out in vision or sound only, and I saw no examples of any region ever doing this intentionally (to put a fault caption or similar to air). The opts back to the Channel 4 break fillers in some regions looked a little more
“manual” than an up and down fade through black, but often they looked like hard cuts from and to the black events at the start and end of the commercial break - so if they had a standard gallery, they didn’t always make use of the functions available to them.
My memory is very hazy now, but I saw the Channel 4 area at Tyne Tees in 1987. It was a bank of about 4 monitors which showed the incoming raw C4 feed, their outgoing feed to the transmitter, and screens for previewing and cueing up ads. The “control panel” seemed to have a number of buttons but not faders (if I remember rightly - it was so long ago!)
Cut out the coupon in your TV Times!