ASUS Zenbook Duo is a dual-screened laptop. It might be bigger than you are looking for, but it is exactly that form factor.

No. GrapheneOS only supports Google Pixel devices. I do not know if the Boox boot loader can be unlocked to support any custom ROMs.

I remember seeing a lot of reporting referring to an NSA best practices guide recommending to reboot your phone weekly. They do not really go into much detail though. Either way, it definitely does not hurt to reboot.

Here is one of the articles that I found: https://www.forbes.com/sites/daveywinder/2024/06/01/nsa-warns-iphone--android-users-to-turn-it-off-and-on-again/