California Health Care Directive

Can I appoint someone to speak on my behalf, even if I am already able to do so through an attorney or advance health care directive in the State of California?

Also, I can use one of these documents to enable them to make mental health and / or drug treatment decisions for me?

If you are able to make a decision about your care, you should inquire about the good VRS. bad options. Why let someone else decide for you? You can ask your friends and family members for input. If you were so ill or mentally incompetent that you could not, you can get one. Most judges want the individual to participate as much as possible and then leave the Guardini / POA decisions as far as treatment.
You can find someone to speak on your behalf, but you'll have to convince the judge that's what you want.