I most frequently take the daytime flight as I fly up in the front and enjoy the heck out of the meal and drinks.
The window dimming is entirely up to you. JAL does try to make an initial adjustment on their flights so as not to inconvenience others if you drift off with the window open, but if you want it open that is entirely up to you in business and first. I cannot speak to the policy in economy.
I have also always been in full control of my window blinds on JAL. I also see the centralised dimming, but I have never experienced it to be locked after. I have some great pictures from north of Canada and of Ellesmere Island. Though when flying in December it was anyway dark, and no real difference if the blinds were open or not.