Part 1 is just an intro, the rest are much longer. For saving, you can use the quicksave key, then if you press quickload it'll load into the map, though the game will forget that it's a workshop map. That just means the vote screen, lightboard and things in the pause menu don't show up, but the map itself will work fine. You can also use "save blah" and "load blah" in the dev console to save/load to a specific filename, then you can be playing as many maps as you like simultaneously. Quicksave/load just uses those commands with the filename "quick".