Desktop mode for the Google Pixel 9 Pro XL Is built into the OS out of the box but you will need to enable it from Developer settings. USB c to HDMI on the the pixel 9 XL is pretty straight forward and allows us to mirror the built in screen to an external display but enabling Desktop mode from deve...
Probably not. The barebones desktop mode has been integrated into Android core for quite a few years now. So it is safe to assume that it is there in GrapheneOS. But yes, someone should confirm that.
I can confirm that it is present in LineageOS.
The option is there on my Pixel 8 Pro with GrapheneOS, but I have no idea if it will actually work, as I've never had a need to use it. But at least we know the option seems to be available.