I think the literal physical stars are referred to. The centaur also thought the stars would go out:

"So the wandless have become wiser than the wizards. What a joke! Tell me, son of Lily, do the Muggles in their wisdom say that soon the skies will be empty?"

"Empty?" Harry said. "Er... no?"

"The other centaurs in this forest have stayed from your presence, for we are sworn not to set ourselves against the heavens' course. Because, in becoming entangled in your fate, we might become less innocent in what is to come. I alone have dared approach you."

"I... don't understand."

"No. You are innocent, as the stars say. And to slay something innocent to save oneself, that is a terrible deed. One would live only a cursed life, a half-life, from that day. For any centaur would surely be cast out, if he slew a foal."

Literal stars. Literally torn apart. The sun must be tamed. And more distant stars are also dangerous.

"End" can also mean "goal". This is somewhat interesting in light of EY's work at MIRI.

I like this, and you can resolve "HE IS THE END OF THE WORLD" by combining the rule that prophecies are meant to be heard by those they affect and to cause their consequences, and that to Voldemort, his death is the end of his world. So HE IS THE END OF THE WORLD is true for Voldemort, because Harry's killing power can be the end of Voldemort's world.

I don't think this is likely, if only because of the unsatisfyingness. However:

And the messages would come out in riddles, and only someone who heard the prophecy in the seer's original voice would hear all the meaning that was in the riddle. There was no possible way that Millicent could just give out a prophecy any time she wanted, about school bullies, and then remember it, and if she had it would've come out as 'the skeleton is the key' and not 'Susan Bones has to be there'. (Ch.77)

Some foreshadowing on the idea of ominous-sounding prophecy terms actually referring to people's names.

Beneath the moonlight glints a tiny fragment of silver, a fraction of a line... (black robes, falling) ...blood spills out in litres, and someone screams a word.

"blood spills out in litres" meshes well with "TEAR APART".

And then, since he's already figured it out when guessing about the Comed-Tea, Harry will make himself believe that the stars are the people, and so the prophecy means not him, but Voldemort, which is why if he does not stop Voldemort right now, he will fail to fulfill the Vow, which is impossible.

So the Vow will work in his favor, possibly boosting his abilitiesto misdirect in Parseltongue.

Also, Voldemort is aalready the end of the world, and has been since possessing the Pioneer Plague (which can be reached by phoenix at any moment).

Also, Voldemort put on broomstick enchantments on his bones, and Harry already knows a way in which they are fallible: CRAP. NEWTON...

How do we know Sirius is present? Because there is a Death Eater named "Mr Grim" who is stated to have known the Potters.

Is Sirius a Death Eater in this fic? In canon, he was thought to be one, but he never was.