it fairly simple bumper rush means someone has a lvl 40 char in normal and can bump you if you get to a5 to next level. YOu only get the Baal quest to move on and can go back and do summit later.
Crush is done in regular diablo not LOD. so only 4 not 5 acts.
Once you are crushed to hell via a bumper if get to completion of a4 hell convert char to LOD if wish to now be able to do all a5 qs for your char.