sorry for the slow reply!!!
the distance from the inside surface of the top panel to the top edge of the motherboard (rampage iv gene) is 64mm. it looks like the nepton would fit with one fan. probably not two. you're right that the thicker 140mm rad i was contemplating will conflict. i hoped it would be in front of the mobo, but it won't. will need a thinner rad.
the 140mm fan that came with the corsair h90 AIO plus it's 140mm radiator leaves about 6 or 7 mm to spare before the motherboard. it fits well.
sorry but i don't know the type of screw. next time i take it apart, i'll compare to some various metric screws i have here. i'd guess M3.
what they should have done is a) shaped the PSU bracket slightly differently and b) located the power extension cord elsewhere so that a 240mm rad would fit at the bottom. with a mATX motherboard, there's a bunch of extra space there, it has really nice clearance below to the cool air at floor level, and it has filters.
this drawing is how i ended up configured for now:
i got better temps with all of the top 3 fans as an intake. the very top fan as intake helped CPU temps under load and the two 80mm fan as intakes particularly helped the CPU temps when the GPU is under load. the stock configuration blows air from the bottom over the GPU (as much as makes it past the fans of the ACX cooler) which presumably gets hot. adding fresh cool air from the pair of 80s helped, at which point i switched the bottom two fans to exhaust.