You can do this now and as long as the Camera is being used, it doesn’t come back on. If you switch away from the Camera and RIGHT back, it retains Night Mode off. If you switch away for a longer time (or kill Camera), it’s active again.
Thing is, there’s a setting for Photos that Preserves Creative Controls (filter, aspect ratio, light/depth) rather than automatically reset. I think that setting should also turn off Night Mode, but it doesn’t. I’m on the beta, so I’ll enter that as a bug
I know it doesn’t work for younger folks, but I’ve shown older folks the results I get from Night Mode and now they happily sit or stand more still to capture the look! Then they say “That’s a good pic! Send me that.” LOL