I have noticed many descriptors of the time, sky and moon in the story recently. I think they might be a clue.
At the Quidditch match:
"June in Scotland meant plenty of daylight; sunset wasn't until ten."
"As the sun set and Harry started using Lumos to read his books"
"And as the stars began to come out"
"Harry glanced at his watch - eleven-oh-four at night. Harry was now reading a sixth-year Transfiguration textbook; or rather he'd weighted the book open, illuminated by a Muggle glowstick,"
At the graveyard:
...
Agreed. However, if we are in the mirror (or being mislead about location in some other way), I would expect things to make more sense after coming to that realisation. So far, they don't.
I'm trying to think up all my other confusions, and other evidence for mirror scenarios to try to make it all fall into place.
The mirror seems too mysterious to have finished its role in the story - Harry can understand more of the false words of comprehension, but he hasn't twigged yet. What could understanding "I show not your face but your coherent extrapolated