DA is short for District Attorney. After an arrest, the DA decides if there is sufficient evidence to prosecute and which charges to charge the suspect with. In Vivian Dimitri's case, the DA would have to decide to try to have Vivian declared insane and sent back to the mental hospital or tried for murder (and attempted murder).

Mrs. Columbo, if everything that we hear about her is true, would be an amazing woman. She can't cook, but she has too many hobbies and interests to mention. She even reads the shipping news (The Conspirators). My personal theory is that Columbo often makes things up to help him dig up info. With Tommy Brown for example, Columbo didn't know Brown's music, so instead he says: "My wife can't get enough of it" (if my memory serves right). That gave Columbo an excuse to ask for a live recording that provided a clue.

The situation with Dr. Steadman was tricky. I'm sure that the doctor doesn't want to see anybody murdered, but he cannot explicitly warn Columbo of the danger without breaking doctor/patient confidentiality. I like the way he danced around the rule without specifically mentioning Vivian.