The Philips monitor will have a manual brightness control. Have you found its buttons? They'll (usually) be at the bottom or the right-hand side of the screen, sometimes hidden around the back.
https://www.damiensymonds.net/2015/04/the-handyman-method.html
But I hasten to add that I don't think there's a problem. I genuinely can't see what shadow you're talking about?
You can't, sorry. You'd need to try to clone the blanket afterwards.
Gain is the important one to adjust, as far as I know. As I said in the write-up, every screen is a bit different so it's hard for me to help with that part.
Tad cool like "I want to try calibrating again"? Or "I can live with it"?