When using rear fill you want it to sound like reflected sound. Nothing else.
The huge delay is to make it sound like there is a large room behind you.
The L-R or vise versa is to remove any mono or center content. Using this content will pull your ears to the rear.
In the end what you are trying to achieve is the simulation of hearing reflections on a much larger room than a vehicle and all the little things we do to process the rear fill are to eliminate the rears from sounding solid, but instead washed out and being reflected to you from a distance.
Have you ever been to a concert?
If you have you'll here the direct sound of the band coming from in front of you but you'll also hear the room the band is performing in.
It comes in lower in volume. It's basically an echo but continuous and much faster.
It's this sound we try to achieve with rear fill. Not to sound like the band had speakers at the back of the room which is what non processed rear fill does.

Also why band limited and not full range. Reflected sound looses high end and low end content pretty quickly. High end content especially gets duller faster over distance. The more high end content you remove the further away the back wall of the fake room we are trying to create will appear.