I finally watched it last night and thought the whole episode was awesome. "Carter" plays a chump on Once as well, so loved seeing him get his dues towards the end. I'm not sure what else they could have done with the zombies. Leaving them there as the hoard grows wasn't ideal, and they can't really kill that many of them off. I was thinking back to the prison and how they had fencing up to just kill them one at a time, but that would take forever. The comic eventually has them luring herds off away from civilization, so cool to see that on the show already.
Obviously they went this way to support the plot, but I think maybe they could have diverted the hoard sooner or immediately got them moving in another direction. They had a car, so why not just use that horn as the noisemaker instead of all the gunshots and flares? In fact, why not use the horn to offset that other noisemaker at the end? They should have just used all that construction equipment to make a big pit and lured them all in there. I didn't really understand the big rush, but it was all fun and exciting.
As for Morgan and the Bo Staff, that was a really interesting, deliberate question from Rick on who trained him. I wonder if Morgan is really on his own or scouting for some other group. This might be how they bring in *SPOILER* Jesus, who could have found and trained Morgan. Interested in seeing how that one plays out. As for the horn, my money's on the Wolves. They made a point of showing that Wolves Are Near sign at the beginning and those guys are obviously well experienced with controlling/redirecting zombies.