There is no such thing as a "shotgun casing". There are, however, shotgun shell (properly known as a shotshell) casings.
When Mathias enters the Sherriffs office, he catches the swing door that the camera crew just opened a second earlier filming him as he walks through the location.
Cady speaks to Vic about confidential information she learned while working at the law firm. What she saw and was concerned about was a lawsuit that had been filed between two companies. The plot involved her not being able to disclose the lawsuit, however lawsuits when filed are public record and can be obtained by anyone via the clerk of court in the county where the suit was filed.
When Walt is confronting Connolly (Branch's Dad) about staging the false conditions of Branch's murder/suicide, he says that Connolly had Branch's truck towed back to his house, and left the scene on a mountain bike. Assuming that he TOOK Branch's body and the bike down to the river in the truck, and had it towed away later, but when first they found the Murder/Suicide setup, the Deputy says that she found Branch's Truck parked 100 yards downstream.
Vic says someone tried to frame Walt by planting the 'Rainier' beer can as if no one in the town would know the brand of beer he drank.