I understand how it works, with using different items there really isnt a gamemode, just a minigame - but what about the loading screen and MVP etc?
you dont understand - a lot of the code between the items and addons are integrated to such an extent that you cant just toss them into other maps and expect them to work. they also occasionally make assumptions about the build or have quirks regarding how the code is written that requires bricks/names/events to be set up a very particular way.
this goes for the loading screen and statistics gathering, which is used for MVP. you wouldn't be able to repurpose any of that code/assets since it would all be designed around the server.