Sadly, I've had to discover that really, the best thing to do with dementia is defuse, defuse, defuse. Agree. Placate. Don't argue. Be a pillow BUT... and this is important.... have those limits and boundaries.
"I want to drive my car!"
"Ok, well, let's look into that tomorrow."
"Now!"
"Ok, let me make a phone call."
And so on. It sucks, but... that's the advise from experts.