I think Fuji uses a Sony sensor, so it uses the same technology as that of the Aptina patent acquired by Sony.
It's a dual gain as explained here : . https://www.photonstophotos.net/Aptina/DR-Pix_WhitePaper.pdf
Yes ...In the case of a native dual ISO (Panasonic), the dynamics offered via each of the two native ISO concerned, is exactly the same ( "X" stops above the medium gray, "Y" stops below)
I d'ont know ...