So according to what gold told me above, new mappings can only be implemented by a lead developer, and then it can be loaded/unloaded by any admin?
No, only rcon admin can reload the script. Not every admin have that power anyway.
The map file update doesn't need a GMX, means it can be done without GMX. Yes, you can be there if it's not an interior. You will fall off during reload, if you are in interior.