On Thursday we have to take my Aunt to the doctor, no matter what. Do we go in as "Mr. Nice Guy" or "Commando"?

Follow the doctor's orders and be prepared for a firestorm. Can the car be "stolen"..parked out of sight? Or "taken in for a recall" that will take forever to repair? Let the local police know the situation.

Well she did not show for dr. So we met with him instead. He invoked the HCP, reported her to Elder Protective Services and told us to take her keys away or disable her car. Any suggestions now?

You go in stealth mode. I would have mom sitting facing the MD and let her do the talking. I would stand behind her. If she told a lie about how well she was doing, I would silently shake my head "no". The doctor was really good at picking up on this and would ask her for details. She gave herself away, so I was not the bad guy.

How does she get to the doctor?

SSSepe. it always helps to star off as Mr Nice guy otherwise someone's hackles go straight up and you get no co-operation. Then if you don't get what you want ask the Dr why they won't help and finally get the machine gun out. After that change the Dr.
You can access mini mental competence tests on the web. if you can get away with that it can at least give you some idea of what is going on.
Can you get her to the Dr on the pretext of an annual physical. medicare likes people to have them plus she probably needs a flu shot.

She is 79 years old and is DEFINITELY showing signs of Dementia/Early Alzheimer's. She often forgets conversations, places she has gone, bank transactions she has made, etc. We are trying to step in to help, as she was never married nor had any children. We are her next of kin. We legally have no right to her medical records, yet we do have a POA and a HCP. However, she refuses for us to take her to the doctor, because we are hoping to get a mental status check on her.

Why are you asking this question? What is the situation?