Note: I’m not super good with UE4 and I don’t know the capabilities but here is my “go” at trying to answer your questions.
1.) I’d say with something that size, and with that many textures, polys, and information. You’d get some client side FPS drops, and probably server side lag as well. So my suggestion is try level streaming. Make several maps about 1km x 1km then stream the levels to where there isn’t loading screens.
2.) You’d have to setup LODs or get the culling (I think that’s the proper word) to make it easier on user/server systems.
3.) You’re more than likely going to have to have shards/instances where the whole world is loaded, or within view/draw distance to have an X amount of players. I’d say it’d be better to have shards/instances than not having them.
I had a thought, the TES games do it, I’m sure others do as well. Where when you have buildings that are able to be entered, when they active the door of it, it throws the player into a loading screen and puts them in a level of the interior of that building. To where on the outside it looks like an actual building, but it’s really just a facade. Then when the player loads that “level” it is only loading/rendering the people that are on that level, which the only thing they’re rending is a very minimal interior. On a scale of what I’m guessing you’re thinking, that might be a lot of work, but it would help with performance.
When I create my singleplayer game, that is pretty much how I’m going to have it. Where when a player enters a building they actually load a whole new map.
Hopefully I kind of sort of helped you. I tried my best to answer. And welcome to UE4 and the Forums!