That's a pretty sweet build.Huh.
1: My case is the Phanteks Pro M Tempered Glass edition. It is quite spacey, and there are no visible cables except from the PSU shroud to my GPU. Everything is zip-tied nice and neat, and I even got a cablemod with combs and such to make it all pretty.
2: I have 4x Noctua NF-P14s Redux, two in the front for intake, and two on top for exhaust. The rear exhaust is a Corsair ML140. (My theme was Black, Grey and White inside, so I needed at least two White LED fans, CPU and rear exhaust in this case.)
3: I can! ... And have. My fan curve is pretty agressive since noise doesn't bug me. I usually have a fan or something going anyways... Complete silence drives me insane, I like ambient noise.
My full build is here still: System Builder - Core i7-8700K 3.7 GHz 6-Core, GeForce GTX 1080 Ti 11 GB SC2, Enthoo Pro M Tempered Glass (Black) ATX Mid Tower - PCPartPicker
Put this thing together...last April/May? It's been a year already wowie.
For your problem: 90°C on a 8700k at 4.7GHz sounds a bit too high for me too. I know that they are running hot, but 4.7GHz should be manageable.
Just in case, I'd try to refit my cooler and reapply thermal paste. Maybe the cooler isn't making full contact for whatever reason. I doubt it it is going to change anything, but it won't hurt either. I'd also try undervolting a bit and see how it goes.
If nothing helps the good, old solution of throwing more money at the problem remains. I'm big fan of Cryorig and the H7 is a great performer for that price point, but better (more expansive) options exist. You are occupying all memory banks, so ram clearance is a problem here though.
But the Cryorig R1 Universal and the new Noctua NH-U12A could be able to help you out without interfering with the ram. Especially the new Noctua cooler (with the new fans) is getting pretty great reviews so far.