Short of writing the letter and letting the DA know you will not testify against your boyfriend, there is not much you can do except wait to be abused again.
Mr. Fink does not practice criminal law and does not seem to understand the criminal justice system.
Only the District Attorney can file criminal charges, and only a judge can drop the charges once they are filed.
In many counties, the DA will proceed with a prosecution even though the alleged victim doesn't want to. If you made statements to the police that could be used against your boyfriend, but you testify to something different at trial, your original statement may be admitted as evidence. The District Attorney would argue that you were telling the truth in the first place, but changed your story to protect your boyfriend.
You should cooperate with your boyfriend's attorney, who is in the best position to know what you can do.
Writing a letter to the DA will probably not accomplish much. The DA is likely to ignore your pleas to drop the case; and assuming you appear after being properly subpoenaed, they will prosecute. Contact your boyfriend's defense attorney to discuss what actually happened that night and your current intentions. The lawyer can also explain what constitutes proper service. A subpoena is not enforceable unless it has been properly served.
If you gave a false statement to the police, you will probably want to talk to your own lawyer at some point before testifying in court. You have a constitutional right not to be forced to incriminate yourself. If your testimony will cause you to have to admit to committing a crime (i.e., filing a false police report), you can assert your 5th Amendment privilege and refuse to answer any questions that would cause you to incriminate yourself. You will be provided with a court-appointed attorney if you financially qualify.
No, you cannot drop the domestic violence charges against your boyfriend. In California, a crime is viewed as an offense against the people of the state. The "victim" is legally just another witness, with no power to bring or drop charges.
That being said, however, the victim's desire that the charges be dropped may have an impact on the prosecutor's decision whether to proceed. In most cases it does not, however, and the victim will be dragged along for the ride, even unwillingly.