As a preface, while the best course of action isn't always clear and the reality of implementing some solution is often rocky, the bipolar diagnosis situation you describe is probably the single most common situation in which legal arrangements must be made for an adult child, and is almost as common as the need for children to make legal arrangements for the care of their declining parents.
You aren't the only one going through situations like these. Probably 0.5%-2% of people in any given area experience bipolar, usually starting in adolescence or young adulthood, and difficulties managing it of the kind that you describe are the rule and not the exception.
Also, just ignoring the problems you describe is a very bad idea. Premature death either from suicide or bad judgment related to the bipolar diagnosis is all too common in these situations. It is serious business, not something that should be thought of as bad character, or futile to do anything about, or blameworthy. It just is, and if someone doesn't do something when the plan gets off course, serious consequences often follow.
There are really several intertwined issues present here.
Realistically, given the nature of the concerns expressed, a limited guardianship may be necessary to accomplish the goals expressed.
What Are Medical Powers Of Attorney?
A healthcare or medical power of attorney gives the person who holds it (who is called an "agent" or "proxy") the authority to make medical decisions for someone called the principal (i.e. John Smith) when the principal lacks the capacity to give informed consent at that very moment to do so.
This is because a power of attorney is an inherently revocable document expressing the wishes of the person writing it. You generally can't bind yourself in the future without court approval or a contractual relationship with a third party which a power of attorney is not.
It isn't uncommon for medical personnel to decide on the spot when to and not to listen to someone with a medical power of attorney based upon how mentally competent the patient seems at the time on a decision-by-decision basis. For example, they might defer to the medical power of attorney agent when the patient is unconscious or heavily drugged, and listen to the patient when the patient is conscious, not drugged, and not acting erratically.
Parents, incidentally, do not automatically have this authority, nor do spouses.
A medical POA is a document that allows the agent to say "yes" when the patient (i.e. John Smith) cannot.
Another name for a document that is very similar and sometimes used is a "health care proxy."
It would typically cost a few hundred dollars to $1,000 to have a medical power of attorney drawn up after discussing the situation and the principal's needs in a meeting with a lawyer and might take an initial meeting and then a second one at which the document is signed after it is prepared following the initial meeting. Other lawyers might manage this in a single meeting and draft it while you wait.
If all you need is a power of attorney, don't be penny wise and pound foolish by doing it yourself, unless the form is provided to you by the health care provider you will spend most of your time dealing with and they prefer their own form.
Otherwise, the likelihood that you will have to pay more to a lawyer later cleaning up your own mistakes probably exceeds any money that you will save.
Other Kinds Of Authorizations
Many medical providers will allow someone to act on behalf of a patient in matters other than matters that call for the kind of medical decision that would normally require the informed consent of the patient, even when the patient is not manifestly incapable of making medical decisions at that very moment. This could simply be a note in the file that the patient has given that person authority to do so, it could be a written authorization to access HIPAA protected personal health information of the patient, and it could be a variety of other things (e.g., authority to make financial arrangements). Some of this is often incorporated in the same document as a medical POA.
Picking Up Controlled Substances
A Medical POA may, or may not, necessarily be sufficient to authorize someone to pick up a controlled substance on behalf of a patient if the patient is physically able to do so, without the presence of the patient. I don't know what the true rule of law under the controlled substances acts and pharmacy regulation is, but I do know that practice in real life varies quite a bit. The best practical solution to the issue of picking up controlled substances would be to ask the usual pharmacist what they require and to comply. (A legal guardian would generally have the power to pick up controlled substances for a ward.)
Guardianships and Limited Guardianships
What Is a Guardianship?
A guardian of the person is someone appointed by a court who has the authority to make medical decisions and other personal life decisions for their ward (i.e. John Smith), even contrary to their apparent stated wishes.
A guardian has the authority to say "yes" and also to say "no" to the expressed wishes of the ward, overruling the ward. A guardianship of an adult can be general, or can be limited on a customized basis.
A guardian must be appointed by a court with jurisdiction over these cases, usually in the county where the ward resides. But, a guardianship can be requested by the ward as opposed to contested.
Realistically, a court would be unlikely to grant a full guardianship or a contested guardianship in these circumstances, but might grant a limited guardianship with the consent of the ward in these circumstances. The parents and possibly any siblings, would have a right to notice of the proceedings and to object or to seek to be appointed instead.
What Process Is Involved In Having a Guardian Appointed?
This would realistically be a proceeding that should ideally involve a specialist lawyer (with experience in mental health or elder law and guardianships) and at least one medical professional's statement (probably a treating psychiatrist or psychologist). There would also probably be a court investigator or guardian ad litem appointed at the ward's expense, to confirm that the facts represented in the petition to have a guardian appointed really reflect the ward's intent. Usually, a proposed guardian selected by the adult ward during a lucid interval would have priority for appointment.
The medical professional and lawyer should be able to provide good suggestions regarding what the scope of the limited guardianship needs to be, although don't ignore or fail to give full credit to your own layperson's practical understanding of the situation either.
The guardian would have to provide information to the court in connection with the petition showing eligibility to serve (e.g. criminal record check, credit check, CV, nomination by ward). Often the guardian would have to demonstrate good intentions towards the ward in some way, especially if the guardian is a third party and not someone who serves as a guardian as a livelihood. Some courts would require the guardian to have insurance for liability in connection with the task or a surety bond up to some dollar amount.
The final decision would usually be made in an in-person hearing at which the ward, the proposed guardian, the proposed guardian's lawyer, the medical professional, the guardian ad litem or investigator, the judge, a court clerk, a court reporter, and any family members who chose to appear (with their lawyers, if any), were present. If the guardianship was granted, perhaps with modifications requested by the judge to the terms of the guardianship, then the Court would issue what are called "Letters" that formally appoint the guardian to the post.
Once appointed, the guardian would have to file periodic status reports with the court and would also be subject to the court's jurisdiction in the event of any future dispute regarding the guardianship, or any allegations of misconduct by the guardian, or any circumstance that requires court approval such as a change in the terms of the guardianship or in the person serving as guardian.
The procedural details I am describing are approximate and aren't necessarily up-to-the minute correct, and might vary somewhat even from court to court within California under local rules and customs of practice; but they give you a gist of what the process would be like if it is working properly and with best practices.
Typically, this might cost $3,000 to $10,000 all in for an uncontested proceeding, and many times that much in the event of a contested attempt to have a guardian appointed.
Health Insurance Eligibility
I'll defer to someone else's answer regarding health insurance eligibility, as I don't have time to look into that at the moment. My instinct is that this wouldn't be a problem in any case except a guardianship and probably wouldn't be a problem even in a case with a third party guardian, but I can't confirm that without doing research.