If he traveled far, he'd have crossed numerous bridges over rivers. Did he pick this one because it has a separate bridge for each lane, or because it was close and the first one he crossed. He did seem hurried. They think he or an accomplice dumped the cases while moving. I also think he painted the suitcases because he thought they were identifiable as coming from Nichols Discount City, who hired local manufacturers for their discount goods, and maybe he or they had a connection to the store, perhaps a nearby location in PA.
And the shot to the neck: I'm guessing it's to help remove the head. Again it seems like he's in a hurry. You'd think he'd only use a gun in an isolated area but I still think it's possible he could have got away with firing a gun inside a room in town.