How do I show the pcp how far the dementia has progressed in such a short visit?

You inform the PCP that she/he is not qualified to judge the level of dementia present nor to do the proper testing to test for it, and that this must be done. You begin a diary of daily activities you see, actions you see. You keep it short and sweet, incident after incident and give it to the neuro-psyc specialist. Some of what you are describing may come under "psych" diagnosis. You need to know what you are dealing with, have it thoroughly tested and staged. If that doesn't work then you make yourself the greatest pest they ever came across.
After Covid leaves you can make ER visits until you get the testing done. Until then, not wise to do.

If she's actually been screened for dementia by her PCP, she should be referred to a neurologist for more definitive tests. You can show the dr how far dementia has progessed by journaling her behavior. Many patients act fine when visiting their dr., but you will have your observations to prove otherwise.

Your Mom need a neurologist. Dementia is out of the PCPs experience. A neurologist will test her. He may order labs to rule out anything physical. She should be checked for a UTI. Write down what you have seen. I always did my notes in 14 font and one page. Short and sweet. What she is doing when she seems OK in front of people for short periods is "showtiming". If her symptoms get worse in late afternoon into the evening, thats "sundowning".

This is what my list was like:

1. Shows signs of sundowning
2. Shows signs of showtiming
3. repeats the same thing over and over.

Give ur list to the receptionist and ask her to give them to the doctor. This gives the Dr. a basis for his questions to her.

Hda5678, I responded to your other, very similar question. The tick may be caused by her medication, if she's had any new additions or changes in dosage. You can take her to a different doctor if you feel they are not seeing what they should. Did they even give her a cognitive exam? The one that involves drawing a clock and putting the time on the hands? And remembering the 3 words? My MIL had "apparent competency". This is what the social worker called what was going on with her: if you asked my MIL general questions like, "how are you doing?" "What's up?" etc. she would give back answers that seemed appropriate. But when you asked her specific questions like, "What's today's date? Who is the President/VP? What season are we in? She could not answer the questions. We were missing her symptoms for a long time because of this. Video your mom being asked these questions and then take that to her doctor if necessary. Please remember that if someone has medical PoA for her they do not need to wait for a medical diagnosis to provide help for her or even apply for Medicaid. A facility administrator can interview her and be able to determine if she can manage in AL with some extra services (like medication distribution) or MC.

I would start keeping notes of everything your mom is doing that you feel is not "normal." Then you can either present the Dr with those notes prior to her appt. or send them to him or her via their patient portal.
I had to do that all the time with my husband, as like your mom, when his Dr would ask him how he was doing, he would always respond, fine. But thankfully his Dr knew otherwise, as I had sent him via the patient portal, what really was going on.

There are also places that will test a person for any signs of mental decline. You might want to look into that. Her Dr. should be able to recommend such a place, as they're usually affiliated with your local hospitals.(at least they are here where I live) Good luck.