Look at some questions you asked. They don't require over-analysis. "13 .How do UVs, and materials work?" UE4 already has a big document about this. https://docs.unrealengine.com/latest/INT/Engine/Rendering/Materials/ You really have to read this stuff first and start making something out of it. I know it sounds desirable…
I'm creating a single player only game, that is high intensive along the lines of GTA/Arma, with a 30x30 km map, and about 220 km of it actually be populated (meaning it's the land mass of the island), with 3-5 areas of it with HIGH density objects/assets. I want it to work on 4 GB computer from like 2018/2019 on lowest…