In California criminal trials, the prosecution goes first and the defense last for both opening and closing arguments. Barnes should not have been able to have the last word.
After Drake does some investigation for Mason, he (Drake) hands Mason a note with the street address of a diner on 4th Street in Carmel, California. The street address includes a street number, but there are no street numbers in Carmel. Because of this, residents must go to the post office to pick up their mail, and businesses are identified simply by the street they're on and sometimes by the cross street as well.
A defense attorney would have redirected his client into emphatically stating she did not kidnap nor did she murder her child, even though these were the outcomes of her having an affair.
It's implied that Hamilton Burger is helping Perry Mason with the trial in order to embarrass his boss and clear the way for Burger himself to become District Attorney. The DA states he plans to run for Mayor off of the fame which a successful conviction will garner him. If he had succeeded, he would have had to vacate the DA's office, allowing Burger to move up without the need to resort to underhanded methods; however, there is no guarantee that Barnes would have won an election to the mayor's office.
In the fantasy scene where Detective Ennis is testifying in court, Perry Mason is seen in one shot holding exhibits of evidence in both hands. When he points to Ennis in the next shot, his left hand is empty.