Excuse me for a long post.
Just to be clear, nobody can force a doctor to act to provide any medical treatment that they don't think is beneficial. So if the doctor doesn't think a treatment is of benefit, they won't give it. This is how, for example, DNACPR decisions are made - resuscitation is a medical treatment and it's up to a doctor whether they think, if your heart stopped, trying to restart it would a) work at all and b) would result in anything other than an utterly shit quality of life. Hence your dh rocking up to a mental health hospital and saying 'here's my wife, she's got those funny hormone changes going haha, lock her up for me chaps' isn't going to happen. Apart from anything else, it takes stratospheric levels of illness to be admitted to a mental health unit in the UK, and tbh you can see why, as they are profoundly untherapeutic places, it's just that sometimes the alternative is even worse. (A sore point for me as my late dh was assessed as low risk and admission refused, 3 hours before he took his own life). It's also possible that you're mixing up American law with UK law - in general, AFAIK, US family members have more of a legal role in treatment decisions than UK ones.
What everyone has the right to do is REFUSE treatment options. And that can really matter.
It's important to understand as well that an attorney should make decisions in YOUR best interests - not theirs. So, for example, you are 89 and have dementia but you still enjoy life and unlike most people on these boards (unlike me) you have always said you want to keep going as long as possible. You get an infection, as people with dementia do, your GP goes to see you at home and sees that you arent very alert, you're not really swallowing so oral antibiotics are going to be a struggle, discusses hospital admission or urgent community response for IV antibiotics with your dh. Now, if it were me, I want no truck with anything to prolong my life once my mental capacity is shot for any big decisions, and I expect my dp to refuse IV antibiotics in any setting and for my GP to work on a purely palliative basis. But maybe you feel differently and your DP knows this. So he says yes to the IV, you have a few days of them, and you pull round for another few years, though you aren't as mobile as you were before the infection.
My Advanced Directive is on my GP notes. It's based on my son's age, and on the purpose of treatment. Nothing to prolong my life in any way, once he is a certain age (and I've lost capacity). I don't mind for example a feeding tube if that's the only way to get painkillers into me (eg I'm so dehydrated that my veins have collapsed). They can do what they need, provided the aim is palliative. I also specifically ask for a review by a palliative consultant, but I'm aware I can't demand that and expect to get it.
Ask yourself; if you don't trust your dh, why do you trust a sequence of ever-changing health professionals who've never met you MORE? Some will be great, some average, some not so great. The point is that you will not be able to decide. It really does happen, however difficult it is to conceive of it, that you can't make choices or communicate them.
You can always include others as a balance. But don't create a 'balance' by adding an attorney who hates your dh. Someone to support him, maybe.