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:
"The moon above was over three-quarters full, already seeming bright with night not fully fallen."
"gleaming darkly beneath the fading twilight sky"
"A tall form rested upon the altar, and even in the dimming twilight it looked too pale."
"Red eyes gleamed beneath the fading twilight,"
"on a twilight-lit stone altar."
"The twilight sky had dimmed further"
"but the moonlight was too faint for certainty"
"Harry saw by the moonlight that they all now lay in another heap by the altar"
"The gibbous moon riding higher in the cloudless sky, the stars and wash of the Milky Way visible in all their majesty within the darkness"
If it is fully dark, it must be well past 10pm, as the text says the sun sets at 10pm. Harry left the quidditch match shortly after 11.04pm to prepare for his quest, so he could be missed or quidditch-disrupting-events could happen any time after that. Despite this, we are assured the cavalry is not coming. If it were past 11pm, what else would have to be true to know that the cavalry isn't coming?
So I did some research.
Definitions of 'shades' of twilight as shown in chart
To my surprise, my research indicates that the night never gets to 'astronomical twilight' in Scotland on the night of June 13-14. The sun sets for several hours, but doesn't go more than 12 degrees below the horizon. The sun needs to go more than 18 degrees below the horizon for the silhouette of the horizon to disappear and to be able to see the fainter stars with the naked eye.
Given all the focus on images of the stars in this story, I expect Harry, Voldemort and Eliezer to notice the difference between "the stars and wash of the Milky Way visible in all their majesty within the darkness" and the dimmer stars being missing because it is not dark enough. What does this indicate?
Are they not in Scotland? Are they not in June? Is that the real sky? Are they playing out some astronomy fan's wish fulfillment in the mirror? (Harry is already very confused that Hermione was resurrected.)
Harry's interest in astronomy enables him to notice that it shouldn't be this dark - it won't have been for some days or weeks now. Can he test any of the above without getting in too much more trouble?
This may also be a genuine mistake, my reading too much into things, a spell Voldemort cast for the ambience, or something else, but I thought it worth considering. Any thoughts?
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 volition." help with now?
Dumbledore has learned not to cave to the terrorist's demands - seeing Harry as a hostage, I expected Dumbledore to trap them both.
Hermione was resurrected awfully easily.
I will go looking through other people's solutions for more evidence.