How are those 2 vertical support bars connected to the main lateral brace? I think I may be missing those.

they physically bolt down to the chassis. You have to remove a piece of foam that is molded under the brace behind the seats (it's mainly there for sound dampening and to reduce chassis vibrations from the CBE, and it comes off easily, no breaking anything). Once you take out the foam, you'll see holes to bolt it down to