Bummer.
Ok, so much for the workaround. Let's go back to the original problem. Have you tried (for testing purposes) using the "PDF Presentation" function with any completely different files?
That looks right to me? I even downloaded your screenshot, and copied the loupe area and shrunk it down so it fitted on the real eye, and toggled the layer off and on, and it looks identical in brightness.
Once you've done the raw processing, use a gradient layer to replace the whole background: https://www.damiensymonds.net/preventing-banding-in-backdrops/
Please, @Cleyvosier? I'm trying to figure out why you're considering buying another (used) screen. Is it because you can't get either of your current screens to calibrate accurately?
Oh, I'm so sorry, I forgot.
For that bit, leave your brush at 100% opacity, and actually change the colour you're painting with (by clicking on the foreground swatch in the toolbar) to 50% grey (128/128/128).
No, it's a pressure sensitivity setting.
See the little icons to the left and right of the "Flow" setting in the options bar? One of them controls it, I can't remember which.
I would try a Hue/Saturation layer for the water. Check the "Colorize" box, and try these values:
Hue +185
Saturation +35
Lightness +15
Then fiddle to your personal taste.