Why didnt Anderson hear the kid? Because Anderson went through the door and Shepard doubled back and looked into a vent, where the kid was a good 4 feet in, whispering to Shepard while the outside world was being blown apart.
Why didn't the Reapers defend the beam? They..they did. They sent down Reapers and Harbinger himself to stop you, there were only 20 or so marines on the ground rushing and 2 bursts of the laser almost killed everyone instantly.
How come Shepard didnt evade the laser like on Rannoch? Because it states that Harbinger is in a class of his own as a Reaper, possibly the first Reaper ever, his laser has no delay on it and is immensely powerful, not to mention you fought a Reaper destroyer on Rannoch which is more a ground warmachine and not a full fledged Dreadnought.
Why did people retreat? Jesus Harbinger just blew up the entire "this is all our nothing" rush. It's understandable that panic would sweep whoever was left over.