: - Is there no real/tangible aspect of the Mantle? It's the
: responsibility or guardianship of life, but if it's just an empty concept,
: why is it worth waging galactic genocide over, especially since the
: Precursors aren't around?

Most of the details of The Mantle are unknown at this point in time. That's one of the cards that I guess 343 is holding closer to their chest for now. Why it was made, why it is necessary, why it's so important, etc etc. It's still pretty mysterious, kinda like the beings who penned/pen it (the Precursors). I can assure you, though, that's it not a mere empty concept. The Precursors don't throw a dart in the dark and choose their Inheritors based on random chance. But again, we don't know much at this point.

I will say this, though: I am pretty sure that the Precursors ARE around. That they never left. And that they guide and direct everything towards their own ends. And I would even go so far as to say that there is a direct link between their guiding and manipulating of events and John's "luck."

: - How does this square with Guilty Spark's blunt "you ARE Forerunner"
: in Halo 3?

It doesn't. And that's because there are two sets of Halo lore: Bungie's and 343's.

Under Bungie's lore, Humans and Forerunners are one and the same race.

Under 343's lore they are merely a cousin species to Forerunners, with both species being made in the likeness of Precursors.

343 re-interprets all information that points to Humans being Forerunners in whatever ways that they need to in order to keep their lore boat afloat.

Specifically, under Bungie's lore, Humans and Forerunners are directly related, kinda like an ancestral relationship sort of thing -- we are of their lineage. When it's said that "Humans are Forerunners" it isn't like you can take some random dude from an office cubicle in Cleveland and claim that when you're looking at him, you're seeing what a Forerunner would look like.

My understanding is that they were sort of a really cool imagining of what humanity could become, if we ever got our act together and made tons of progress over thousands of years. I guess imagine genetically ideal human beings, physically both flawless and maximized (there's so much more to it, but that'd be where you start). Put another way, imagine the ultimate fulfillment of what a human being could be, and then imagine a whole race of people like that. In fact, when I first saw Destiny's "The Awoken" in the character creator, and chose the bald versions of the characters, I got a really familiar feeling about them, and with some degree of modification, I'd say that they come pretty close to what some Forerunners might have looked like under Bungie (maybe minus the purple-y and green-y skin tones and such). They are sort of like our forefathers, but there's some interesting unexplained stuff about how exactly we had that relationship with them.

Under Bungie's lore, it seems like when Librarian first discovered pre-historic humanity on Earth, it was an unexpected surprise.

Imagine if Americans suddenly found a bunch of cave-men (or something similar - the point being that they're "early humans") living in the middle of nowhere, somewhere out in South Dakota, doing their tribal thing, completely isolated from the rest of the world and unaware of what was going on outside of their little bubble. Imagine how surprised those Americans would be to be to find those cave-men. I think that's kinda-sorta what it was like for Bungie's Forerunners. Sad that it happened so close to the end of their war with The Flood, when they couldn't really do much with that information. (Another comparison might be to the main characters in Stargate SG-1 and Stargate Atlantis, who discover many other human beings on other planets, completely alien in culture and history.)

As for citation on these points...

1.) Fast-forward to 2 minutes and 2 seconds here:

2.) Forerunner AI Mendicant Bias (the most advanced AI that the Forerunners ever built) speaking in the language of the San'Shyuum to the nearby members of The Covenant, as shown in Joe Staten's Contact Harvest, pages 274-276.
(Or fast-forward to ~07:15:20 in this audiobook):

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"< FOR EONS I HAVE WATCHED >
[...]
< LISTENED TO YOU MISINTERPRET >
[...]
< THIS IS NOT RECLAMATION >
[...]
< THIS IS RECLAIMER >
[...]
< AND THOSE IT REPRESENTS ARE MY MAKERS >

Now it was Fortitude's turn to feel weak in the knees. He grasped the arms of his throne and tried to come to terms with an impossible revelation: each glyph represented a Reclaimer, not a relic, and each Reclaimer was one of the planet's aliens -- which could only mean one thing.

"The Forerunners," the Minister whispered. "Some were left behind.""

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3.) Bungie's own David Candland, confirming these claims:

One of the most striking retcons to me is the basic concept of whole role of humanity. Originally (back in Halo 1) the reason why humans weren't conquered and incorporated into the Covenant collective was because their presence defied Covenant religion. When the Covenant discovered humans, they knew they were forerunners, but their presence implied the "great journey" failed. They also weren't the all powerful gods they worshiped, so the Prophets wanted to "sweep them under the carpet," as it were.

The plot lines in our games imply this everywhere - the chief being called reclaimer, only humans being able to retrieve and insert the index, Spark telling the chief, "you are forerunner." etc.

I just finished reading Cryptum this morning. In it I discovered that the forerunner are now an entirely different caste-based species with Humans as a beaten, but allied race. At this point, I just follow Cody's lead, shrug my shoulders and say, "eh. it was a good read anyway. I'll buy book 2."

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4.) Bungie's Librarian speaking to Didact, as seen in the Halo 3 Terminals, where she refers to how special humans were in the eyes of Forerunners.
(From here: http://haloterms.bungie.org/ )

"Additionally, when you access this terminal, during your browsing, you will hear three garbled sentences. These sentences are in fact, backwards and when the sounds are played properly, they become messages, possibly from Mendicant Bias. The first sentence appears when you initially access the terminal and when reversed says "Archive Accessed". The second sentence appears when the screen turns red and the first message fragments, it says, when played properly, "Interruption". The third sentence appears when the terminal is proceeding to the fragment 3/7 and reads "Lineage Confirmed". The first messages meaning is obvious, that you have accessed the terminal. The second message most likely refers to the fragmenting of the first document accessed and the screen turning red. The final message is probably a reference to the Humans', Forerunner lineage."
(From here: https://halo3archives.com/terminals/3/ )

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5.) From the Iris ARG:

"The anomalous world is in a perilous location beyond the line. The secrets it holds must be preserved, plans within plans within plans. The inhabitants; these unique denizens, must be researched. They may hold answers to our own mysteries. What irony that we discovered this treasure, only at the end of things. But what fortune that we still had time to save them. The thing we built on that world will vouchsafe their lives, but perhaps one day it will be used for its intended purpose. If the plan succeeds, and they are saved, it will be a good world. If the plan fails, and the adversary succeeds, it will remain an enigma forever with no one left to reclaim it."
(From here: http://blog.ascendantjustice.com/halo-3/the-terminals/shediedineden4/ )

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6.) Forerunner sentinel speaking with Spartan-III Ash-G099, as shown in Eric Nylund's Ghosts of Onyx, page 133.
(Or fast-forward to ~03:41:01 in this audiobook):

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""Security protocols enabled," the drone spoke over the COM.
"I understood that," Ash replied.
"Ring offensive system activated," it said. "Shield in count-down mode. Exchange proper counterresponse, Reclaimer."
"I don't want to hurt you," Ash tried.
He had no idea what this thing wanted.
"Non sequitur," it said. "Reclassification of targets as non-Reclaimers. Aboriginal subspecies. Collect for further analysis -- else neutralize as possible infection vector."

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Once again, if you have any further questions, I'm happy to help. :)