Nfrahm1

try putting in each 4gb stick one at a time to see if one of them is dead?

it's used as a spacer in some laptops so the ssd isn't sitting directly on the chassis, and will allow easier connection to the sata cables

1. the difference is '80 plus' certification. i, personally, do not put my hands on anything that is NOT certified but that's your preference
EDIT:oops i didn't see that they're both 80 plus certified
2. modular vs semi modular is a highly personal choice. if cable management is a big issue for you, then get  the semi modular because things will look a lot neater inside your case. non modular will come with a big jumble of cables.
3. 550w should be just fine if you're not overclocking. but to be safe, a little higher would be better.
 

you said he installed pubg immediately, did he skip installing drivers for his GPU?
 
EDIT: also what kind of lag is it? input lag? server lag? low frame rate lag?

noctua's thermal paste is very good, just like their coolers and fans.
it should be the stuff that came with your cpu cooler. NT-H1

I'm running an asus strix 1080 ti and 7700k @ 5ghz and i still get 90-100 fps. I don't play the game very much anymore but i jus lived with it because i didn't really notice the difference considering i have g sync as well, so the frames were always buttery smooth
EDIT: i'm also running the same resolution as you