Assuming no immediate crisis requiring inpatient care (no concern for safety of patient or others), wouldn't the best practice for administering a complete evaluation/assessment of dementia/memory loss be in the context of the person's natural environment? The intent being to define the baseline for a variety of skills such as self-care, finances, orientation etc. My research has led me to believe that this scenario would be the default for obtaining the most valid results and that the APA (and AAN) guidelines would suggest this is the case.
Someone who has POA can act on behalf of the principal, following the principal's wishes. For example, if the principal wants to sell stocks and give the money to charity the POA can do that. But if the POA doesn't think that is a good decision and wants to protect the assets for the principal's own care, then (depending on the terms of the POA) it may be necessary to prove that the principal is no longer competent to make his/her own financial decisions. Depending on the terms of the POA, the attorney in fact may then be able to make decisions or it may be necessary to seek guardianship.
Evaluating someone's level of dementia for purposes of advising on levels of care needed usually requires input from the people who have been providing care. When my husband had his annual visit to Mayo clinic and spend half a day taking tests, I was interviewed during that period and answered a long questionnaire about his activities and abilities. And then together we spend a couple of hours with the behavioral neurologist who administered other tests and simply talked to us.
Considering all the people involved, I don't think it would have been practical for the evaluations to take place in our home. We traveled to the clinic. But I don't know that it would have made any difference to the outcome.
Having a highly skilled neurologist running all over the countryside to see patients in their home environment to assess their dementia just isn't practical, in my opinion.