1. DA = District Attorney. He/she is the lawyer (attorney) employed by
the district to prosecute. You can think of him/her as the lawyer
of the government to prosecute.
2. Yes, you can think of it as "in all situations". In other words,
don't pretend to be what you are not (for example, don't pretend
that you are an expert, or don't act differently with the intention
of trying to impress others).
I am curious to know the equivalent term for DA that is used in UK. Thanks