It is an interesting topic. The reality is that an individual will always be encouraged to express their wishes regardless of being deemed to no longer being deemed to have capacity. The dementia journey of course is not like a light switch that turns off everything but a longer journey. So for instance, a person may no longer be capable of feeding themselves but can make a decision over what meal they would like upon being given a choice.
I think there is a big difference to between a financial LPA and a Health & Welfare one. Ultimately, subject to the legal responsibilities of a financial LPA, the LPA has significant control. A Health & Welfare LPA is far more restrictive, particularly the more advanced stage of dementia an individual is in, given the more active involvement of healthcare professionals.
Given historical and ongoing family dispute issues, when I have been approached regarding DNR, I actually chose to effectively hand over that responsibility to the healthcare professionals with mum's advance wishes and my feelings recorded within it. This has been carried out several times now, once at home, once in hospital and then upon move to a Nursing Home. For the latest one, I advised what my mum's advance wishes had been, my personal feeling that mum's dementia had progressed to sufficient a level that if she were to have CPR with the inevitable breaking of ribs, and likely further deterioration that I didn't feel CPR would be appropriate and if successful likely cause further suffering. The first one carried out, which was several years ago now, I felt that it wasn't yet time for a DNR given mum's issues at the time really were mainly down to short term memory issues.
I also expressed that upon 'end of life', unless a hospital visit was likely to help (eg intravenous antibiotics for infection), I felt mum should remain in the nursing home with any appropriate pain medication. This has been recorded and will be included with medical assessment should that situation arise.
Personally I think the main benefit of having the H&W LPA is the legal benefit of the access to all records, eg medical and nursing home. Without complete insight I think it can be much harder, if not impossible to make decisions.
My understanding is that unlike financial matters, where you can apply to the Court of Protection to get a deputyship awarded rather than LPA, that it is very rare for the Court of Protection to do so on Health & Welfare LPA's. And even with a health & welfare LPA, other family members will always be included where possible in such as best interest decisions unless ultimately Court of Protection deem an individual's involvement is not in the best interests of the individual.