The NYPD has a well-documented long-standing issue with their radio systems being unable to communicate between officers underground and control centres above ground.
At the time this episode was made, this had yet to be resolved, except in parts of the Subway system.
Marcus would not have been able to summon medics and an ambulance without going back up the stairs.
When Marcus and the two officers are walking to the front door of Mr Harris's house, only one of the officers is carrying a firearm, but both have their hands on their holsters. Not a goof: After Marcus pounds on the door and says, "Something's wrong," both officers draw their weapons. The officer in the foreground has large hands that cover much of the weapon, but the butt is clearly visible as he steps onto the porch.
As Marcus descends the dark staircase at Harris's house he uses a torch rather than flick the clearly visible light switch.
Colby's butcher knots are not enough to tie someone to murders.