This is not unheard-of. Some people even have to take their screen down to 0 to get a match.
However, such a low value does prompt me to worry about your room lighting. Have you read this?
That's right!
First, I duplicated the Background layer and ran Dust & Scratches filter at 3/10 (adjust that to your taste). Then masked it to the problem areas only.
Then Handyman, exactly as you said.
I think you are wrong, yes.
In the manual, about 6 or so pages in, it talks about it.
https://www.download.p4c.philips.com/files/b/bdm3201fd_75/bdm3201fd_75_dfu_eng.pdf
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.