inflex2

I used around 15 wired mice in the past 6 months, swapped for logitech g pro wireless and it was the best decision I ever made. 

You can assign different programs to different output devices, so you could assign the game to the headphones, and Spotify etc. to something else.

That one seems to be for people that have two PC's and one headphone/microphone, so probably not.

I'm not all too experienced with headphone amps, I'm more a speaker guy. @rice guru could help you though
 
The switch is made by a company called Little Bear, And I use some of their gear for controlling the volume of my studio monitors, it's simply made, but works great. 0 signal degradation issues.

That switch would work perfectly fine, it's just swapping the signal between two different outputs, not splitting the signal, so no problems there.

Yup, should be fine. 
 
No. A SSD will be fine to throw the OS and OBS on. 
 
16 is more than enough. 

You will lose the warranty in either case (watercooled or air cooled) because you will need to remove the shroud, backplate, fans (air cooled one) and pump (watercooled one).

You are buying a 2500$ GPU that is already watercooled, that you will then watercooled with a custom loop, that's absurd. So a 1200$ 2080Ti makes more sence and saves a lot of money.

That's a complete waste of money. 
 
You don't need separate PC for streaming. 
 
I play and stream/record on one PC and can maintain stable 720p60/1080p60 on stream/capture and games still run close to 200FPS.

Okay, there is a ton if misinformation going on in this thread and I venture to guess that anyone giving their input right now hasn't attempted to stream with a second PC before. 
 
There are pros and cons, but generally if you have the ability and money (which you clearly do here) the pros outweigh the cons IMO. 
 
As for the Streaming PC itself. Diminishing returns are certainly a factor here. 3900x is on the high end but you will be able to render at a very slow h264 preset. (slower is better) Anything more powerful you probably wouldn't notice a difference. Any GPU you add will just be for video out to display essentially so no reason to go overboard here. A $20 GPU will be fine. Use the 1070ti for something that matters.
 
Skip the capture card. NDI Scanner and NDI OBS Plugin is able to send video over IP with virtually no compression or latency.
 
Be sure to take into account a good sound mixer. I find this VASTLY superior to any software option.
 
 
ALL THAT SAID. The 9900KS can CERTAINLY encode and game at the same time and there isn't a need for a streaming PC in this set up at all.. I just prefer the flexibility of a second PC.