How to set up a dev environment?

[Entih]

No, putting the map into the main game's map folder is a silly solution because of the issues it presents when adding other content (which must also put into main game folders, then re-located at time of release and put into the custom story folder. it also means you need to edit things like the primary game english.lang file to enter in text entries, which is a bad idea period).

Just make it such the game does not attempt to auto-load a map at all. Go into main_settings.cfg and set ShowMenu to true, then just load the custom story through the custom stories dialog, but do mind that the custom story is required to be properly set up for this to work.