When Quincy is talking to the men down in the cabin on his boat, the clock on the wall shows no movement of time.
When Quincy and Monahan go to the apartment of the fake Hopgood, Quincy should have told Monahan that since he knew the real Hopgood, he knew that the other guy was an impostor and they therefore had grounds to arrest the impostor for interfering with a criminal investigation and covering up a murder..
Lt. Monahan makes the common mistake of confusing a missing body with a lack of a "Corpus Delicti". In law, the term does not refer to an actual body, but to the corroborating evidence that shows that a crime has been committed.
It's unlikely, if not forbidden, for Tom-Tom, a person known to have a drug problem, to be allowed to work at a hospital where he has access to drugs.