1) Yes.
2) It can Bitstream ATMOS to an external processor for decoding and playback.
3) Yes.
4) A little bit, yes. At a minimum you would need to be willing to search and read forum posts to work it all out. But there are people doing what you want, who could help.
5) Not really. It is all in the Wiki and Forum.
I would be hesitant about the Mac Mini as well. A Windows PC is more likely to work with less trouble, and a discrete graphics card is almost mandatory for serious 4K with HD audio, even if only to support the latest standards.
Finding a local JRiver expert sounds like the right idea, or a remote person who can specify the hardware to be set up. 4K with HDR and ATMOS/DTS:X is still a bit bleeding edge.
But, if you have an external ATMOS capable processor, have you tried to get the Apple TV to Bitstream ATMOS to it, rather than play it?