Alien spawn points are just the starting positions for aliens on the map. Every map needs to have them to allow to spawn (add to the game) aliens. I just don't know where alien spawn points are located for basedefence mission. If on the ground, most probably there is no problem, but if in some facilities (base modules), which your base lacks currently, there is a problem because aliens do not have spawn points to spawn.
The loading screen for basedefence mission is ok - it just puts the info about all map tiles which are loaded for this mission (this will most probably change in the future because map tiles in loading screen is useless information from the user point of view).