#2 and #3 are related. The goal is to keep the airflow of heat coming out the FP from following path of least resistance. Heat rises, so it will exit the FP and go UP to the lid riding the lid until it exits at the top of the left side. The plates help direct the airflow. Moving the exhaust lower also does this.
#4 The fire basket allows you to put a lot more charcoal in, also keeps it from rolling out the vent, also maintains an air vent beneath the lit charcoal.