Legal Question in Criminal Law in California

No. Many witnesses don't want to testify. That's why subpoenas were invented.

In some extreme situations, a witness might persuade a judge not to make her testify. Those situations are rare. And the procedure would be a formal motion, not a letter to the judge. The D.A. would have an opportunity to oppose the motion. And you would need to make a very persuasive argument, showing not just that you want to be excused but that the law says you shouldn't be forced to testify. If the D.A. followed proper procedures and has a legitimate reason to call the witness, the court would almost certainly deny the motion.

A more promising option may be to ask the D.A. to excuse you. It probably won't work, but it might.

No, because it does not afford the defendant the right to cross examine and face the witnesses who will be testifying against him or her.