Yes, two 240s are possible, but I would only use a mini ITX board, not a micro ATX...
There is not enough space between the case and the glass for the fans, they have to be mounted inside the case.
No, the housing is designed for a MicroATX format mainboard.
The mainboard you mention is an ATX format.
https://www.nzxt.com/products/n7-z390-matte-white
Yes, that fits. The max. length of the graphics card may be 300mm. Further information: https://www.corsair.com/de/de/Kategorien/Produkte/Geh%C3%A4use/Crystal-Series-280X-RGB/p/CC-9011137-WW