I've done 3 FormD T1 builds so far, two with a 5800X3D and the Black Ridge. While I'm no expert, I can answer from some experience.

That board supports the 5800X3D with bios version 1.90 or higher, so it just depends on which version it shipped with. Assemble the board/CPU/RAM before installing and test boot. If you get a black screen and nothing else, you'll most likely need to get the bios updated.

The Black Ridge should work fine on that board. As shallow as it appears, you may not even have to remove the stock m.2 heat sink. You do not need to get any extra backplate, it will mount fine without it - just follow the instructions.

I used the slim Noctua 92mm chromax fan mounted to the bottom surface. No reason to "flip" it, you want it to pull air from the outside down through the fins. Using some foam pads under the corners seemed to help with noise a little.

Regular low profile RAM like Corsair LPX is perfect.

The Corsair 12VHPWR adapter will be fine. It's probably a little long, but there's room to bunch up any excess.

If you have any issues with the build, feel free to DM me. I build a lot of PCs and enjoy helping people out when I can.