Well actually I suggested you to replace , not to add new ones , or may be I expressed myself badly ... you need to edit the item.db file , and substituite the objects there with the new cas files you prepared for high mid or what u want to havelike lods , then u need to place the new cas files in the correct buildings folder , remember that the walls need a separate subfolder for it , I think you can see the correct placing by unpacking the models buildings pak and so see how those are organized .... then u need to delete the relative items from the folder (well this isn't actually needed if u edit the db file ) but if u didn't edit the db file u need the game to recreate it I think (this soulution I didn't try ) but if I am not wrong if u delete the item db file the gamwe should generate a new one provided that he finds all the correct buildings in the correct place , u need to follow then the DJ tutorial for that other way by unpaking in the correct sequence all the stuff.... so as you see there are two solutions , or u follow the DJ solution in his tutorial with my small add on at the end of it , or you edit the item db file possibly with an editor or hex editor ....

About the invisible building , it quite never happened to me , but could be for bad export?