The tip of your torch shouldn't be so far in. The brass will melt if it's in the chamber and the forge is working right.
Second, not sure that you'll get enough BTU from that torch, even with MAPP.
Third, 6.5cm(I use 67mm) is ideal for the chamber to insulation size on a 4.5" square brick. (I love mixing standard and metric!!!)
Fourth, the torch angle looks much better in the last video. Wholly inadequate in the pictures and first video.
Fifth, resistance to heat isn't the same as insulation. Seeing that you easily carved the bricks, I'm going to figure that they are insulating. Another way to be sure is if they are heavy or light. Heavy bricks don't insulate, light ones do.