It worked for me - It might or might not for you..... but you have to start blaming al lother authority as opposed to your own (or your mother's). So when your Dad is annoyed about, eg. decisions about driving you must say something like -
"It doesn't really feel fair, Dad, as I know you have been driving even very recently,.... but it is the memory clinic doctor who has ordered this. It is out of my or Mum's hands. We know how awful this must feel for you...but I think they said you would be breaking the law if you continued to drive. I know, I know - they haven't seen you drive...yes it certainly feels unfair. There is nothing we can do about the decision they have made." (You get my drift?) This is strictly speaking not a total lie as I am fairly sure that after a dementia diagnosis it becomes illegal to drive.
I never had to do this about driving with my mother.We had managed to stop that a bit earlier but I did have to convince her regarding her medication taking, which she was doing wrongly(and dangerously!). So after her diagnosis, the memory clinic consultant advised me to do the sensible thing regarding the administration of my mother's daily medication which was, as she lived alone, to get in carers to give them to her...and my mother knew that this was coming out of her bank account too! It was all very wrong and unjust in her eyes because she said she was "perfectly capable" of doing her own medication. But, as safety must always the main priority, even a little lie is justified. Good luck.