It is, I think, never a decision of a substitute decision-maker. There is no room for substitute decision-making in this context. It is the individual who was saying what is to be done to them at a point at which they have lost decision-making capacity, and the clinician assesses the objectively assessable conditions because you've sorted that out by writing down your written request. You figured out what will work. It is something that clinicians can assess, and they determine whether those conditions have been met or not.
There is no substitute decision-making here at all.