As they talk on the pier, Goren and Eames are in shadow, in fact the entire pier is in shadow (the sun is very low in the sky), the camera changes views and a few seconds later when it turns back to Goren and Eames, the sun is clearly much higher in the sky so they are now in full sunlight.
While interrogating the hit-man Detective Goren takes out his sidearm to demonstrate how the killer held his weapon in an amateurish and unprofessional manner. Police officers are not allowed to carry a weapon when they are in an interrogation room with a suspect, it was a huge violation of NYPD regulations for Goren to have his sidearm with him, especially considering that it was clearly loaded.
ADA Carver says that grand jury testimony is not admissible at trial. This is not necessarily true, there are certain circumstances where a person's grand jury testimony can be read into evidence at trial. If a witness is unavailable to testify at trial, either because they are dead or they have fled the jurisdiction and are unable to be located, then their grand jury testimony can be used at trial in place of them testifying.
Onlookers are clearly seen in many of the scenes in this episode. Two times at the pier: the guy with the hockey gear is constantly looking back at what is happening and when the hit man is brought to the pier there's a guy walking towards Goren and Eames. Just when you can almost see this person and he's almost to Goren and Eames-he makes a sharp turn to exit the shot. The scene outside the courthouse where Goren is explaining Dempsey's plan, there's a guy who is standing behind them (Goren, Eames and Carver) who is just smiling in a black beret-looking straight at the camera crew as they walk off to the left.
The "onlooker" at the pier with the hockey sticks was Detective Goren, he was working undercover at the time which is why he kept looking back at what as going on.
When Goren is talking to the victim's husband shortly after the murder, he picks up a large photograph and shows it to him. The boom mic is clearly reflected in the glass of the picture frame.
When showing Eames how the gun was fired from below, Goren has his finger on the trigger while waving the gun in the air. Any one with actual gun training would know to keep your finger off the trigger until you're ready to fire.