Hi josepicos,
Regarding "never getting the success message" - take a look at the circular TAC display at the top left corner of your screenshot. It shows a purple "blob" which is the goal for the mission.
The mission goal always shows as purple in the TAC. Bombing the stuff which shows as red triangles in the TAC will not achieve the mission - the purple stuff needs to be damaged to the extent required in the mision.xml file. Of course if the mission is written in such a way that the goal cannot be achieved for some reason, you will never get a success message.
As for swapping out AA guns in a facility file - go to the Facility folder and find the relevant xml file. You can edit it using a simple text editor like Notepad, or something more powerful like Xml Wrench.
When you open the facility file you will see entries for example "#armor_AA_mobile#1". The a_50cal_quad is in your vehicles folder and from memory, in the xdp file it is in the armor_aa_mobile group.
You can simply put in the xdp name of the 50 cal quad into the line replacing "#armor_AA_mobile".
Once your facility file lists some specific guns, the trouble is that the facility will generate everywhere with those guns - hence you will get German facilities appearing with US guns. When the random elements are used (using the # symbol and the generic group type), guns appropriate to the Axis or Allies are spawned for the facility.
For those facilities that are specifically listed in the Global layer, that does not matter so much if they are unique to one country, then you can choose nation-specific guns without the side-effects.
HTH?