@bojandurmic
You can make anything in fps creator invisible, by giving it a pure black texture (0, 0). If you have trouble changing the texture, I have an invisible wall segment and can attach the download, but try it yourself first
About your second question, I don't have that MP, but the default Dark AI seems to work great for me, just use the TF 341 characters with the default Dark AI. Also make sure you are on the latest version of FPS Creator.
I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones.