One final effort... try adding this to your MAPNAME.csv in zone source:

// fog at edge of level
fx,env/smoke/fx_fog_zombie_amb
fx,env/light/fx_ray_sun_sm_short

i'm completely out of ideas now... lol

fx_start()
{
	fx_init(level._effect["zombie_fog"], "amb_long");	
}
fx_init(fx, key)
{
	if(isdefined(fx) && isdefined(key) )
	{
		fx_spawn = getentarray(key, "targetname");

		for( i = 0; i < fx_spawn.size; i++ )
		{
			PlayLoopedFX(fx, 1, fx_spawn[i].origin);
		}
	}
}

There, corrected that for you. Most fog fx need to be looped.

Would auto looped FX be glitched if they where looped again by this function?

Yes, it would multiply itself rapidly until the engine refuses to draw any more copies. If the fx is looping, you dont need to loop it. If the fx has a set lifetime, you need to loop it on its lifetime in script.